Rackspace database resource output is null

current implementation of rackspace database resource
_resolve_attribute method assumes that clouddatabase object's
hostname and href are set to correct values and just returns
them, but that assumption is not correct. With this change,
hostname/href is retrived if it was not set already.

Fixes Bug #1204601

Change-Id: Ifca44237e20210b5ff1fc1b28f1f8408f5672ae4