These options are set in the neutron-api charm centrally, and
this patch allows neutron-openvswitch charm to continue doing:
1, polling_interval
Just used by neutron l2 agents, so neutron-openvswitch charm
gets it via it's relations and set it in [agent] of ml2_conf.ini
or openvswitch_agent.ini(>=Mitaka)
2, rpc_response_timeout
Used by all neutron agents, so both neutron-gateway charm and
neutron-openvswitch charm get it via it's relations and set it
in [default] of neutron.conf
3, report_interval
Used by all neutron agents, so both neutron-gateway charm and
neutron-openvswitch charm get it via it's relations and set it
in [agent] of neutron.conf
Change-Id: I76c0c75d5f3b4fdd1eb3242b53fde2e829fedca5
Partial-Bug: #1685788
6b5273f415