neutron-server systemd service masked even when not installed

The nova-cloud-controller charm uses charmhelpers.core.host.service_pause to mask some services and does so regardless of whether or not they are installed. The result is that e.g. neutron-server is masked even though it is not installed:

root@juju-c1fcea-test-11:/home/ubuntu# dpkg -l| grep neutron
ii python3-neutronclient 1:7.1.1-0ubuntu1 all client API library for Neutron - Python 3.x
root@juju-c1fcea-test-11:/home/ubuntu# systemctl list-unit-files| grep neutron
neutron-server.service masked enabled

While this does not cause any problems per se it is unnecessary and could be avoided by checking if neutron is installed.