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
Merge "Enable to select an RPC handling plugin under Metaplugin"