ML2 Auditing and monitoring

ML2 connection and auditing is the way of testing or auditing the connection
with mechanism drivers for the backends (like Opendaylight, Midonet etc)
The idea is to perform auditing of mechanism drivers on regular time intervals
and based on results from mechanism driver audit raise appropriate exceptions
regarding failure and logging, the framework to utilize the auditing mechanism will be
added to neutron which can extended by mechanism driver to implements fault reporting
or generation of alarms etc on failure

spec is proposed https://review.openstack.org/#/c/589313/