Add a validate method to dir and memory backends

In order to allow all connections to be validated
post-creation add a new method onto the memory
backend (a no-op) and add a new method onto the
dir backend (which checks directory existence) and
raise appropriate errors when validation fails.

Change-Id: Ie476d0c33a970e543ffba21ee8b669ac02258923
Merge "Add a validate method to dir and memory backends"