webadmin: Fixed tenant name bug

Seems like I messed something up in ProviderModel, which caused the
tenant name to not be retrieved from the database when editing a
provider. This is fixed here, in a way that will hopefully render
maintenance easier as further types of tenant-oriented providers are
added.

Change-Id: I42b58137af760f4fe59985b4b7c59460264fa232
Signed-off-by: Lior Vernia <lvernia@redhat.com>