Cleanup Dogpile KVS Memcache backend support

General cleanup of the Dogpile KVS Memcache backends to be simpler
names and remove the 'dogpile_' prefix for the memcache_driver
argument utilized by the memcached backend pivot point. Documentation
updated to match KVS memcached changes.

DocImpact

Change-Id: Ieb368d440e48111d844c6e715ed17dd54ca50802
bp: dogpile-kvs-backends
`find_migrate_repo` improvement

* Added `repo_name` attribute with default value as `migrate_repo`. Allow
flexibly look for migrate repos with different names.
* Abridged logic of dependency from a module location.
* Moved to `common.utils`. Required to synchronize with
`openstack.common.db.sqlalchemy.migration`

Change-Id: I8aa364f1654fb92fba287cd406670b03f8ec3a54
Merge "Cleanup Dogpile KVS Memcache backend support"
Merge "`find_migrate_repo` improvement"