Juju 2.0 provides support for network spaces, allowing
charm authors to support direct binding of relations and
extra-bindings onto underlying network spaces.
Resync charm-helpers to pickup support for new hookenv
tools and add data extra-binding to the charm metadata.
This allows the local endpoint IP for overlay tunnels to
be configured using network spaces.
Any existing configuration of os-data-network is preferred
over the new binding support if already set.
Change-Id: I0e2e3f51106b6c6483f22ce4abd04bcb098b484e
6ee724fc4b