Tidy pep8 errors.

Add gitreview configuration.

Drop registration of default release selector - not required.

Use default version of ceilometer charm in testing now that
relevant changes have landed.

Ensure sane default release is passed to charm superclass.

Correct behaviour for disabling services based on any of the
required states not being set.

Add xenial-pike test target.

Change-Id: Iefcdc1d4870572dae00e71e1b855fb1abf68a4e9