Change volume size property type to integer

Before it was possible to provide a rational number for SIZE, but cinder
would silently round it down, which leads to inconsistency between
stored resource state and actual cinder volume.

Closes-Bug: #1311707
Change-Id: I97b110f9797e2046bd5d2ae7da227b8dfc19059a
Merge "Change volume size property type to integer"