Group Policy: Implicit Policy Driver

(Patch series identifier: GPM-IPD-1)

This patch adds the implicit_policy group policy driver, which is
responsible for implicitly supplying L2 and L3 policy objects when
they are not explicitly provided via the client API.

Author:            Bob Kukura <kukura@noironetworks.com>
Co-Authored-By:    Sumit Naiksatam <sumitnaiksatam@gmail.com>
Co-Authored-By:    Stephen Wong <s3wong@midokura.com>
Co-Authored-By:    Mohammad Banikazemi <mb@us.ibm.com>
Co-Authored-By:    Mandeep Dhami <dhami@noironetworks.com>

Change-Id: Idad6b6f23ec6f690486d8fa2b6c1c730bb33826a