Get updated vol status in volume.api.reserve.

A race condtion was discovered where a nova volume attach
could easily be performed twice for the same volume.  Although
the cinder attach update would fail, this occured after the BDM
updates were made and in essence the attach to the compute instance
had already been issued.

This change simply forces a get from the DB of the volume-ref in the
reserve call and checks if an attach is already in progress.

Fixes bug: 1096983

Change-Id: Ie0e4156d691ee92b6981078ef0ba62f8c4cdf0c8
Merge "Get updated vol status in volume.api.reserve."