Brick connector revised fix for NFS drivers

This change fixes the error that comes
while getting brick connector and attaching volumes
in case of NFS drivers in cinder. The attribute for
mount point base was not passed to attach_volume method
as the connector initialization logic is common for all
types of protocols. It is fixed by populating
the required parameter in the RemoteFsConnector
by extracting it from connection properties. The infrastructure
for fixing this situation for remote fs drivers is implemented
in this fix. The drivers need to override the method
_get_mount_point_base in their implementation. NFS driver
implements it. GlusterFs implementation of the method to follow
in a separate patch. Hence marking it partial fix.

Change-Id: Id61a437fca1870529fa8b85f7fc2f73eae665294
Partial-Bug: #1238085
Merge "Brick connector revised fix for NFS drivers"