Stop abusing state_path in find_config

It makes no sense to look for configuration in the same location where
state is stored. Replacing state_path by pybasedir keeps current
behaviour while allowing us to change the default state_path from
Designate's install location to /var/lib/designate in a subsequent
patchset.

Change-Id: I82fed1ddde0dbc7340a6ad5dcb80dbcba72b8358