Description of problem:

--disablerepo options causes DNF to traceback


Version-Release number of selected component (if applicable):
dnf-0.2.17-1.git6a055e6.fc18.noarch

How reproducible:
always


Steps to Reproduce:
1. dnf update --disablerepo test
2.
3.
  
Actual results:
Traceback (most recent call last):
  File "/bin/dnf", line 35, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 314, in user_main
    errcode = main(args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 99, in main
    cli.configure(args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/cli.py", line 1393, in configure
    (opts, self.base.cmds) = self.optparser.setupYumConfig(args=args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/cli.py", line 1614, in setupYumConfig
    self.base.repos.disableRepo(repoexp)
  File "/usr/lib/python2.7/site-packages/dnf/yum/__init__.py", line 749, in <lambda>
    repos = property(fget=lambda self: self._getRepos(),
  File "/usr/lib/python2.7/site-packages/dnf/yum/__init__.py", line 617, in _getRepos
    self.getReposFromConfig()
  File "/usr/lib/python2.7/site-packages/dnf/yum/__init__.py", line 493, in getReposFromConfig
    self.getReposFromConfigFile(repofn, repo_age=thisrepo_age)
  File "/usr/lib/python2.7/site-packages/dnf/yum/__init__.py", line 440, in getReposFromConfigFile
    thisrepo = self.readRepoConfig(parser, section)
  File "/usr/lib/python2.7/site-packages/dnf/yum/__init__.py", line 517, in readRepoConfig
    repo.basecachedir = self.cache_c.cachedir
  File "/usr/lib/python2.7/site-packages/dnf/conf.py", line 74, in cachedir
    self._make_ready()
  File "/usr/lib/python2.7/site-packages/dnf/conf.py", line 58, in _make_ready
    self._cachedir = self._retdir(self.prefix)
  File "/usr/lib/python2.7/site-packages/dnf/conf.py", line 70, in _retdir
    return os.path.join(dir, self.suffix)
  File "/usr/lib64/python2.7/posixpath.py", line 66, in join
    if b.startswith('/'):
AttributeError: 'NoneType' object has no attribute 'startswith'

Expected results:


Additional info: