Initial implemention of MetaPlugin
Initial implemention of MetaPlugin

This plugin supports multiple plugins at same time. This plugin is for L3 connectivility
between networks which are realized by different plugins. This plugin add new attribute 'flavor:id'.
flavor:id correspond to specific plugin. flavor-plugin mapping could be configureable by plugin_list config.
This plugin also support extensions. We can map extension to plugin by using extension_map config.

Implements blueprint metaplugin

Change-Id: Ia94d2349fb3ce9f121bbd2505324ae6f0c34247a

moshele/old-networking-mlnx