Defaults to 30s (i.e. enabled) but also allows disabling
healthchecks by setting to 0.
Change-Id: I5bb7d362f0d957237e24f79f1f82583661bed470
Closes-Bug: #1890900
* 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