Brick connector 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
for NFS drivers.

Change-Id: I8601326b318f6f8c53a03610f1b4f2bfd14070ff
Closes-Bug: #1238085
Check if dir exists before calling listdir

Changes along the way to how we clean up and detach after
copying an image to a volume exposed a problem in the cleanup
of the brick/initiator routines.

The clean up in the initiator detach was doing a blind listdir
of /dev/disk/by-path, however due to detach and cleanup being
called upon completion of the image download to the volume if
there are no other devices mapped in this directory the directory
is removed.

The result was that even though the create and copy of the image
was succesful, the HostDriver code called os.lisdir on a directory
that doesn't exist any longer and raises an unhandled exception that
cause the taskflow mechanism to mark the volume as failed.

Change-Id: I488755c1a49a77f42efbb58a7a4eb6f4f084df07
Closes-bug: #1243980
Merge "Brick connector fix for NFS drivers"