Ovsdb plugin compatibility layer

Ovsdb plugin compatibility layer

As of now this layer just proxy API calls to plugin and relay the notification back to all the listener.
All the modules who want to consume to existing plugin interfaces, should move to the compatibility layer,
because plugin interface will change due to it's migration to md-sal.

*.* Although this layer is just proxy, but classes are moved under new package name to avoid conflict
with plugin code. As of now net-virt code is changed to depend on compatibility layer.
*.* Add new feature ovsdb-compatibility-layer to karaf-feature
*.* All integration tests are passing

Patch 3: Rebased and addressed reviewers comment
Patch 4: Fix single feature test for compatibility layer
Patch 5: Fixed integration tests
Patch 6: Added api back that was deleted accidently. Comments and pom.xml cleanup.

Change-Id: Id2587914c63651f067dbef1f7ba0bcb22a7a3fd4
Signed-off-by: Anil Vishnoi <vishnoianil@gmail.com>