Add device identifier to replication device

Add device identifier to replication device

One thing that may be needed by some back ends
that are implementing replication is some sort
of back end device identifier.

For managed back ends this shouldn't be an issue
but for unmanaged, it's handy to have some sort
of identifier string.

This patch just adds that to the response examples
in the volume.manager code as well as adds an update
to the dev docs to point it out.

Change-Id: Ic113d905e65c756439425b03035179e65ce0b39e