The change adds an option to the charm to use JUJU_AVAILABILITY_ZONE
environment variable set by Juju for the hook environment based on the
underlying provider's availability zone information for a given machine.
This information is used to configure the availability_zone setting for
Neutron DHCP and L3 agents specifically because they support it
and for other agents (because both neutron.conf and agent-specific
configuration files are loaded) such as metadata agents and lbaas
agents.
Additionally, a setting is added to allow changing the default
availability zone because 'nova' is a default value coming from the
Neutron defaults for agents.
Change-Id: I94303aa70ee3adc6ace0f9af1e7c4f5c0edbcdb5
Closes-Bug: #1796068
71c0120d21