Enable to select an RPC handling plugin under Metaplugin

Multiple plugins under metaplugin become 'q-plugin' topic
consumers and a request from an agent is handled by one of
them randomly. Fortunatly most of RPC callbacks are common
for plugins but a problem occurs if an RPC is not supported
by the received plugin.

This is one of risks when using metaplugin. Fundamental fix
of this problem (such as RPC delegation handling of metaplugin)
is difficult since each plugin needs to modify.
But when only one plugin has plugin specific RPCs and other
RPCs are independet of plugins, if the plugin can be selected
for RPC handling, the problem does not happen. Typical use
case of metaplugin such as combination of an agent-based
plugin and a controller-based plugin often applies to this
condition.

This patch adds 'rpc_flavor' configuration parameter to
select an RPC handling plugin. If 'rpc_flavor' is specified,
only the specified plugin becomes 'q-plugin' topic consumer.
If 'rpc_flavor' is not specified, the behavior is same as
previous one.

Change-Id: If133b054bba53829cebe63c1e0ebe6099eb1fd95
Closes-bug: #1267330
DocImpact
Add update binding:profile with physical_network

Add temporary solution in order to support multiple physical networks
by mlnx ML2 MechanismDriver.
Due to non merged patches in nova that should support propagating
physical_network retrieved from port binding:profile attribute
to VIF/Network object.
The code will be removed once relevant nova patches are merged.
The code is disabled by default and should be enabled via
ml2_conf_mlnx.ini config file.

Change-Id: I815f9e28774efd47bccd1c57481e6ba89075792b
Closes-bug: #1291209
tests/unit: clean up notification driver

Some tests doesn't cleanup notification driver. So some tests can
run against unintended notification driver depending on execution order

Change-Id: I7dbed47153be66684827b2693bbc995dddfb4c1b
Related-Bug: #1281481
Partial-Bug: #1284549
Merge "tests/unit: clean up notification driver"
Merge "Add update binding:profile with physical_network"
Merge "Enable to select an RPC handling plugin under Metaplugin"