Bionic, being the next LTS, is important to enable for dev
and test as early as possible ahead of 18.02.

Zesty goes EOL in Jan 2018. The next stable charms release (18.02)
will not provide Zesty series support, as it was an interim
(non-LTS) release.

Change-Id: If96d593120b3b334819fca892042ec17c7403dd6