Provide FlowCapableNodeDataChangeListener to openstack.netvirt via NodeCacheManager

Provide FlowCapableNodeDataChangeListener to openstack.netvirt via NodeCacheManager

With this change, netvirt will have a thread safe cache of all flowCapable nodes.
This is needed for the lbaas handler, so it can efficiently find the nodes it needs
to configure, w/out using ad-sal switch manager.

Patch 2:
- add missing Copyright headers
- add code review comments

Note: NodeUtils has been duplicated and needs to be moved to .utils pkg
This will be taken care in gerrit: https://git.opendaylight.org/gerrit/#/c/16487/

Change-Id: Id3e98e8e4fb735c4a1cba8e325dd7a1c44a6d192
Signed-off-by: Flavio Fernandes <ffernand@redhat.com>