BP: Provider Validation for L3 Plugins

Blueprint spec on adding vendor validation capabilities
for L3 services for Juno.

Initial work is to separate out the validation logic
so that it can be extended/overridden.

Later work could consider using TaskFlow to handle all
processing steps, but needs additional research first.

Change-Id: Ic34376fe261e36d4341ff7b2f1727f4a897e7479