Add support for Juju network spaces

Juju 2.0 provides support for network spaces, allowing
charm authors to support direct binding of relations and
extra-bindings onto underlying network spaces.

Add public and cluster extra bindings to this charm to
support separation of client facing and cluster network
traffic using Juju network spaces.

Existing network configuration options will still be
preferred over any Juju provided network bindings, ensuring
that upgrades to existing deployments don't break.

Change-Id: I78ab6993ad5bd324ea52e279c6ca2630f965544c