Remove deprecated neutron-lbaasv2-agent from NRPE checks
This patch removes the deprecated neutron-lbaasv2-agent service from
NRPE checks after an OpenStack upgrade to an OpenStack version greater
than Stein.
* 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.
The bundle disco-stein was in the dev bundles because
a bug with the Python 3.7 syntax prevented the success of
the deployment of HA clusters. This bug is fixed, we can
reintegrate disco-stein in the gate bundles.
Other small additions:
- removing sitepackages in tox.ini to avoid test env pollution
- skip_missing_interpreters in tox.ini set to False to avoid false
positives by skipping missing interpreters.