Don't call mapper() outside of declarative registry

Resolve the following RemovedIn20Warning:

  Calling the mapper() function directly outside of a declarative
  registry is deprecated. Please use the
  sqlalchemy.orm.registry.map_imperatively() function for a classical
  mapping.

Change-Id: I92a7ccdd48eedd4c788384033743daf50a9dc113
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>