Support for extensions in ML2

The current ML2 plugin supports only extensions defined in the plugin and it
does not support any extensions in the mechanism drivers. The changes in this
commit allows mechanism drivers to define new extensions.

Change-Id: I28da19fabf6de2e9f0d687f875aaaa24c8bbc4f0
Implements: blueprint extensions-in-ml2
Merge "Support for extensions in ML2"