Description of problem:
When using cache, preping install fails because of yum.conf symlink already
present in cache tree

Version-Release number of selected component (if applicable):
rpm -q mock
mock-0.6.12-1.fc7

How reproducible:
Always

Steps to Reproduce:
1. Create a mock cache like said in MockTricks
2. Try to build a package
3.
  
Actual results:
Traceback (most recent call last):
  File "/usr/bin/mock", line 1046, in <module>
    main()
  File "/usr/bin/mock", line 1043, in main
    do_rebuild(config_opts, srpms)
  File "/usr/bin/mock", line 910, in do_rebuild
    my.prep()
  File "/usr/bin/mock", line 254, in prep
    self._prep_install()
  File "/usr/bin/mock", line 659, in _prep_install
    os.symlink('../yum.conf', os.path.join(yumdir, 'yum.conf'))
OSError: [Errno 17] File exists


Expected results:
Building of the package

Additional info:
A if os.path.exists should be done over the symlink path like it is done for
regular file yum.conf and devices above
Current workaround is to edit the cache to remove the symlink each time the
cache is generated.