* get a list of l3 plugins to enable based on relation data coming from
neutron-api;
* refactor adding fwaasv2 service plugins to the l3 agent to accommodate
the l3_extension_plugins change.
See https://github.com/juju/charm-helpers/pull/370
See LP: #1842353
Change-Id: Ic3a8e302942ed331bc3d80223e123c13d61db3b2
Closes-Bug: #1842353