LBaaS: throw proper exception on duplicating monitor association

Fixes bug 1208728

Change-Id: I6cf27766eedff34a6ed5062a50b049ab0a5bf96a
Add metering extension and base class

This a part of the blueprint bandwidth-router-label

This patch initiates the blueprint by adding base class
to associate labels and metering rules to tenant's routers.

Change-Id: Ia93b49d881e79c3291730cff7b80f26c56fedb48
Merge "Add metering extension and base class"
Merge "LBaaS: throw proper exception on duplicating monitor association"