Fix regression in Rackspace Cloud Servers tests

Commit 63b7f14403 introduced a regression in the Rackspace Cloud Server
tests.  Instance.volumes() now relies on its VOLUME property's default.
Since CloudServer inherits volumes() from Instance but defines its own
VOLUMES property, its VOLUMES property should also have a default.

Closes-Bug: #1265034
Change-Id: Ib17337c5d06aedf65da5497ad48858d649ba4e96
Merge "Fix regression in Rackspace Cloud Servers tests"