Force service provider relationships to load

Force service provider relationships to load

A race condition was exposed in the LBaaS V2 db layer that was caused by a
hack to get around this issue.  The real issue is that since the
ProviderResourceAssociation is inserted independently, any models that were
created before this insert will not have their relationship with the
ProviderResourceAssocation loaded.  Using the session.expire_all method will
force the session to retrieve all new data and load this relationship for any
resource that uses this relationship.

Change-Id: I940b541f4ef9c489126cd2d215b1d857f0624de0
Closes-Bug: #1504465