rgw: Allow an implicit tenant in case of Keystone

rgw: Allow an implicit tenant in case of Keystone

This, unfortunately, introduces possible double lookups, but
they should be cached. Also, the logic appears somewhat convoluted,
although the intent is quite simple: if you're an OpenStack user
with a Keystone authentication, we allow an implicit tenant of
the same name as the user.

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Conflicts:
	src/rgw/rgw_swift.cc