Code review comment for lp://staging/~parthm/bzr/376388-dot-bazaar-ownership

This patch fixes bug #376388 by ensuring the .bazaar,
.bazaar/bazaar.conf and .bzr.log inherit usr and grp ownership
from the containing folder.

Following functions are added to osutils:
* copy_ownership: copies usr/grp ownership from src file/dir to dst file/dir
* mkdir: wraps os.mkdir and adds an optional arg ownership_src. When provided,
  the newly created dir is given ownership based on ownership_src.
* open: wraps __builtin__.open and adds an optional arg ownership_src. When provided,
  the newly created file is given ownership based on ownership_src.
* parent_dir: wraps os.path.dirname handling the special case of dirname returning ''
  by returning '.' instead.

config.py and trace.py are updated to use osutils.{mkdir,open} rather than the
base functions during creation of above mentioned files.

Whitebox tests are added to test_osutils.py to verify mkdir, open and parent_dir.
Manual testing for done using "sudo bzr whoami a@b".

ChmodFeature was added to tests/__init__.py to ensure os.chmod support for test
cases.

This proposal was originally based of 2.0:
https://code.launchpad.net/~parthm/bzr/2.0_376388_dot_bazaar_ownership/+merge/19593/
but is resubmitted now as it is meant to go into trunk and testing requires
overrideAttr support that is not available in 2.0 branch.