Make plugin and l3plugin available as mixin's properties

This makes the code simpler and slightly more efficient.
This is a minor refactoring related to a race condition
observed when the agent is processing a router while its
interfaces are being removed.

Related-bug: #1355409

Change-Id: Ic0ce25aa6a977e28b1ab76081c74da4c46c2ca6b
Merge "Make plugin and l3plugin available as mixin's properties"