MinDisk size based on the flavor's Disk size

When you create an instance from image  the instance disk size
is expanded to the disk size specified in the flavor.

Normally you cannot use the snapshot  for creating an instance
with smaller disk size (flavor)  because it  potentialy
causes a data corruption.

Every snapshots minDisk size should be inherited from the flavor.

bug 1200279

Change-Id: I1d0e909b78db2c863b4fbb54ad9065207f1a0abe
Merge "MinDisk size based on the flavor's Disk size"