- Add Zesty as a supported series to metadata.yaml.
- Turn on Xenial-Ocata Amulet test definitions.
- Sync charm helpers to get Juju 2.x amulet compatibility.
- Keeping Zesty-Ocata Amulet test definitions turned off until the
  metadata.yaml changes propagate to the charm store.
- Note: Revert back to cs:mongodb when it supports current UCA pockets
  https://bugs.launchpad.net/charm-ceilometer/+bug/1671865

Change-Id: I81dd02655d0aa9ebddef189b9e622a6a83beb4ba