Add extension_manager and support for extensions in linuxbridge agent

Add extension_manager and support for extensions in linuxbridge agent

There is extensions mechanism for l2 agents already but it was
implemented only for openvswitch l2 agent. This patch adds support for
such extensions also for linuxbridge agent.

This patch also adds support for network_update events received by the
agent via RPC. It is required because sometimes when a network is
updated (for example with a QoS policy is attached to it) all ports that
belong to the network should also be updated.

Change-Id: Ie81c818d0eb817b044a6df1cbddc5864f118fe3f
Partial-bug: 1468803