- 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.
- Resync tox.ini to resolve amulet unit test failures.

Change-Id: Ia14904339099d84d09a2fce171740459a76a8367