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
Merge "Cleanup Dogpile KVS Memcache backend support"