VPNaaS: Unit tests using policy.conf

Instead of adding override_nvalue() in setUp() for tests that need
the policy.json setting for config, created a new base class for
VPN tests, which derives from the neutron BaseTestCase and overrides
the config for policy_file.

Change-Id: Ia504df51510d964c5f5baccb0b921af1de406f21
Partially-Implements: blueprint service-split
Merge "VPNaaS: Unit tests using policy.conf"