is_deployable doesn't really give any additional information for a
release, it is removed. To make change API-backward-compatible, it is
retained as a property dependent on release.state.
New state is added - manageonly, for environments that are not able to
be deployed, but can still be managed.
Change-Id: I518a0114730a2f227c9ef035a376f9a90d3d5bbd
Closes-bug: #1503303
DocImpact