implement TDSWorker model

Containers that deploy a thredds application should be able to register themselves against the django app (i.e. create something like a TDSWorker model via a POST-request). This is necessary if multiple replicas exist of the same THREDDS-Server (as it is the case in a kubernetes setup). The TDS_TOMCAT_URL should be removed and replaced by a corresponding field of the TDSServer model. Each TDSWorker should be associated with a TDSServer