Only allow one keystone-server node

Allowing more makes nearly no sense, as the endpoint in keystone can
only point to one node.
Use new validation API in validate_proposal_after_save
Stop using validate_has_active_database_proposal

We actually don't need this:
 - if we just save without applying, we don't need to check this
 - if we save and apply but the database proposal is not active, then
   the keystone proposal will be queued (which is the right behavior)
Use new find_dep_proposal helper
Merge pull request #156 from vuntz/validation

Use new helpers to create/validate proposals