Cleanup ReleaseDeployment resources

[jmgilman]

Each new deployment creates an associated ReleaseDeployment resource in the cluster. This resource eventually reconciles to a conclusion but is never cleaned up. The result is that these objects will grow indefinitely.

Instead, a ttl field should be added, which dictates how long the ReleaseDeployment should live before getting cleaned up by the operator. It should be optional and default to a sensible value. Once a ReleaseDeployment reaches a "finished" state (either successful or failed), the operator will check over a predetermined interval to determine whether the ReleaseDeployment should be deleted or not.