Cluster Error On Missing Volume Sizes Unoptimal

if the volume size is omitted on an instance, and volume_support
is true, then the error should indicate that the volume size
is missing, vs. stating that the volume sizes don't match.

Change-Id: I40c807313eaed4fb2f433da1c8a80089e01756fa
Closes-Bug: #1369001
Merge "Cluster Error On Missing Volume Sizes Unoptimal"