Code review comment for lp:~mattrae/charm-helpers/lp1635067

I've tested these changes deploying openstack with MAAS using these steps

# check out neutron-openvswitch charm
git clone https://github.com/openstack/charm-neutron-openvswitch.git

# pull in changes from https://review.openstack.org/#/c/392212/
git fetch ssh://<email address hidden>:29418/openstack/charm-neutron-openvswitch refs/changes/12/392212/2 && git cherry-pick -n FETCH_HEAD

# sync charm helpers from lp:~mattrae/charm-helpers/lp1635067, edit charm-helpers-hooks.yaml 'branch: lp:~mattrae/charm-helpers/lp163506'
make sync

# Deploy openstack with the updated charm. Enable DVR to test routers created on compute where neutron-openvswitch charm is deployed. Replace <bridge> in data-port with the bridge created on the host by juju, for example 'data-port: br-ex:br-eno1'

  neutron-api:
    options:
      enable-dvr: true

  neutron-openvswitch:
    options:
      bridge-mappings: physnet1:br-ex
      data-port: br-ex:<bridge>