add a method for validating the existence of a db proposal

Trying to make the code in https://github.com/crowbar/barclamp-keystone/blob/release/roxy/master/crowbar_framework/app/models/keystone_service.rb#L45 more general so it can be reused in other barclamps and in more methods of the same barclamp

The find_active_database_proposal will be used to validate the existence of an active database proposal before applying a proposal.