Validate that we have one glance-server node after saving

Also do not raise exception from proposal_after_save, but use
validation_error.
Use new find_dep_proposal helper
Change default notifier depending on the existence of rabbitmq proposal

Also make sure to express the dependency on the rabbitmq proposal if we
depend on it.
Use validate_dep_proposal_is_active in validate_proposal_after_save
Merge pull request #167 from vuntz/validation

Use new helpers to create/validate proposals