Enable Ocata Amulet Tests

- 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.

Change-Id: I182fffbbfc6e3d475e0d23c58998c997645b9387

src/metadata.yaml

@@ -15,6 +15,7 @@ series:
  - trusty
  - xenial
  - yakkety
+ - zesty
 subordinate: false
 provides:
   dnsaas:

src/tox.ini

@@ -10,7 +10,7 @@ setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
          AMULET_SETUP_TIMEOUT=2700
 whitelist_externals = juju
-passenv = HOME TERM AMULET_*
+passenv = HOME TERM AMULET_* CS_API_URL
 deps = -r{toxinidir}/test-requirements.txt
 install_command =
   pip install --allow-unverified python-apt {opts} {packages}