OCI Vault: key enable/disable and key version deletion #1550

Follow-up from the OCI Vault review on #425.

Not served yet:

• POST /20180608/keys/{keyId}/actions/enable and /actions/disable (EnableKey / DisableKey), with the ENABLING / DISABLING / DISABLED states. Today a key's lifecycle is ENABLED ⇄ PENDING_DELETION, and these paths answer 404 "unknown action".
• Key version deletion: POST /20180608/keys/{keyId}/keyVersions/{keyVersionId}/actions/scheduleDeletion and /cancelDeletion.
• etag response headers and if-match on the Vault resources, consistent with the other OCI handlers once they gain it.