Add in functionality to set key_mangler on dogpile backends

When a key_mangler is set on the region during configuration, ensure
that the backend has the same key_mangler set on it.  This allows
the backend to calculate a hashed key that will match what the
region would produce.

Change-Id: I6fbf921a10eea5d0cfe4d996d8563c56d3638fac
bp: dogpile-kvs-backends
Merge "Add in functionality to set key_mangler on dogpile backends"