Revert "Removed Overseer URL from config files, now reads from JWT instead"

This reverts commit 9324aef9ad77556a1af5c4506be3895bd8c8a69f.

I prefer it to have get the overseer's URL from the config file rather
then indirectly through the token. Configuring that URL on every server
instance does not save any work since the server's own API auth token
must still be configured anyway.
Reapply parts of 9324aef9 not related to the Overseer URL

Change-Id: I3cd32500ab581e18a6ef5744bfe8884b6d1c5a6c