Description of problem:
When building an iso with pungi, yum repos are set up without an associated configuration file. With yum >= 3.4.3-36.fc18, the string representation of the repos is pulled directly from the repo's configuration file. If the repo does not have a configuration file, the path is None and and exeption bubbles up, killing the ISO build process.

The following exception is from a ppc DVD build but I was seeing the same thing on x86_64. I don't have that exception on hand but can easily reproduce if needed.

removing /srv/pungi/test, one moment...
Building f18
Pungi:INFO: Adding repo fedora
Pungi:INFO: URL for repo fedora is ['http://ppc.koji.fedoraproject.org/mash/branched-20120829/18-ppc/ppc64/os/']
Pungi:INFO: Adding repo source
Pungi:INFO: URL for repo source is ['http://ppc.koji.fedoraproject.org/mash/branched-20120829/18-ppc/source/SRPMS/']
Pungi:INFO: Adding repo debuginfo
Pungi:INFO: URL for repo debuginfo is ['http://ppc.koji.fedoraproject.org/mash/branched-20120829/18-ppc/ppc64/debug/']
Pungi:INFO: Adding repo localrepo
Pungi:INFO: URL for repo localrepo is ['file:///home/f18-private-packages/']
Pungi:INFO: Getting sacks for arches ['ppc', 'ppc64', 'noarch', 'src']
Traceback (most recent call last):
  File "/usr/bin/pungi", line 256, in <module>
    main()
  File "/usr/bin/pungi", line 96, in main
    mypungi._inityum() # initialize the yum object for things that need it
  File "/usr/lib/python2.7/site-packages/pypungi/__init__.py", line 287, in _inityum
    self.ayum._getSacks(archlist=arches)
  File "/usr/lib/python2.7/site-packages/yum/__init__.py", line 779, in _getSacks
    self.repos.populateSack(which=repos)
  File "/usr/lib/python2.7/site-packages/yum/repos.py", line 348, in populateSack
    sack.populate(repo, mdtype, callback, cacheonly)
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 184, in populate
    if self._check_db_version(repo, mydbtype):
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 275, in _check_db_version
    return repo._check_db_version(mdtype)
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 1379, in _check_db_version
    repoXML = self.repoXML
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 1565, in <lambda>
    repoXML = property(fget=lambda self: self._getRepoXML(),
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 1561, in _getRepoXML
    self._loadRepoXML(text=self)
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 1552, in _loadRepoXML
    return self._groupLoadRepoXML(text, self._mdpolicy2mdtypes())
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 1526, in _groupLoadRepoXML
    if self._commonLoadRepoXML(text):
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 1350, in _commonLoadRepoXML
    result = self._getFileRepoXML(local, text)
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 1128, in _getFileRepoXML
    size=102400) # setting max size as 100K
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 941, in _getFile
    **kwargs
  File "/usr/lib/python2.7/site-packages/urlgrabber/mirror.py", line 424, in urlgrab
    return self._mirror_try(func, url, kw)
  File "/usr/lib/python2.7/site-packages/urlgrabber/mirror.py", line 402, in _mirror_try
    return func_ref( *(fullurl,), **kwargs )
  File "/usr/lib/python2.7/site-packages/urlgrabber/grabber.py", line 1126, in urlgrab
    return self._retry(opts, retryfunc, url, filename)
  File "/usr/lib/python2.7/site-packages/urlgrabber/grabber.py", line 1020, in _retry
    r = apply(func, (opts,) + args, {})
  File "/usr/lib/python2.7/site-packages/urlgrabber/grabber.py", line 1122, in retryfunc
    fo.close()
  File "/usr/lib/python2.7/site-packages/urlgrabber/grabber.py", line 1857, in close
    self.opts.progress_obj.end(self._amount_read)
  File "/usr/lib/python2.7/site-packages/urlgrabber/progress.py", line 151, in end
    self._do_end(amount_read, now)
  File "/usr/lib/python2.7/site-packages/urlgrabber/progress.py", line 301, in _do_end
    ui_size, ui_time, ui_end)
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 437, in __str__
    return self.ui_id
  File "/usr/lib/python2.7/site-packages/yum/yumRepo.py", line 374, in _ui_id
    val = config._readRawRepoFile(self)
  File "/usr/lib/python2.7/site-packages/yum/config.py", line 1174, in _readRawRepoFile
    ini = INIConfig(open(repo.repofile))
TypeError: coercing to Unicode: need string or buffer, NoneType found


Version-Release number of selected component (if applicable):
yum-3.4.3-36, yum-3.4.3-28

How reproducible:
Every time

Steps to Reproduce:
1. Install yum >= 3.4.3-36 on the build system and have the same version in the
   source repos for the DVD build
2. run pungi

  
Actual results:
Execption from yum bubbles up, killing the ISO build process

Expected results:
ISO builds successfully

Additional info:
I did an ISO build with YumRepository.__str__() returning id instead of ui_id and the same traceback didn't happen.

I also did netinstall and DVD builds using yum-3.4.3-33 adding the patch mentioned in #852240. The build process completed successfully and the ISOs function as expected after the build.