Updates across the charm and unit tests to switch to
execution under Python 3.
Note that the changes are not backwards compatible
with Python 2.
Refactor use of neutronclient python module to simply
wrap the neutron binary, using the yaml output format
to avoid the requirement for a Python 3 module on
older OpenStack release versions.
Change-Id: Ic26b0dd19a76552481939325963a6c21585dee3c
5f22e621c2