Description of problem:
When using mock's cache, buildroot setup fails if the cache was created by a
different user.

Version-Release number of selected component (if applicable):
0.7.2



Steps to Reproduce:
1. As a user, 'usera', Build a package in mock with 'use_cache' enabled.
2. Let build complete and mock exit.
3. As a user, 'userb', attempt to build a package in the same chroot.  mock
tries to use the cache created by 'usera', but there are permission problems.
  
Actual results:
init
clean
prep
This may take a while
unpack cache
Traceback (most recent call last):
  File "/usr/bin/mock", line 1140, in ?
    main()
  File "/usr/bin/mock", line 1137, in main
    do_rebuild(config_opts, srpms)
  File "/usr/bin/mock", line 997, in do_rebuild
    my.prep()
  File "/usr/bin/mock", line 261, in prep
    self._prep_install()
  File "/usr/bin/mock", line 707, in _prep_install
    yumconf_fo = open(yumconf, 'w+')
IOError: [Errno 13] Permission denied:
'/var/lib/mock/centos-5-x86_64/root/etc/yum.conf'


Expected results:
Permissions should be corrected to allow multiple users to use the same mock
root cache, or if that is not possible, mock should ignore the unusable cache
file and re-create the build root from scratch.

Additional info:
This happens, for example, because I have a machine setup as a plague builder
which calls mock as the plague-builder user.  If I try to call mock from my
account for a build root which plague has created the cache for, I will see the
error.