VPNaaS: separate out validation logic for ref impl

VPNaaS: separate out validation logic for ref impl

Modify the persistence logic for VPNaaS to separate out the
validation for the reference implmentation. This allows
providers to override/extend the validation, as needed.
Additional commits will address the separation for other
providers and for other L3 services.

In addition, the logic that sets default values for optional
attributes is also moved into separate methods to allow
providers to override the defaults used.

Only attributes that have multiple validations are considered
at this time, but this can be extended, as needed.

Currently, one UT fails due to an oslo.messaging bug that
has been fixed, but not yet released for Neutron use.
As a workaround, the affected test case has been commented
out. This test verifies that the right (only, in this case)
device driver is used for validation. Once the issue with
releasing oslo.messaging 1.4.0.0a2 or newer is resolved,
this can be uncommented.

In the future, TaskFlow may be used for the L3 services workflow,
and these changes will provide validation modularization for that
effort.

Change-Id: Id420983ccbceece5f4a00546941618318b9562c7
Partially-implements: blueprint l3-svcs-vendor-validation