DPM config: remove usage of DPM_DAV_ENABLED and DPM_HOST variables

* `DPM_DAV_ENABLED` duplicates `HTTPS_ENABLED`.
* `DPM_HOST` duplicates `DPM_HOSTS` and `DPM_IS_HEAD_NODE` (replaced by `DEFAULT_DPM_HOST` for UI/WN configuration)

Tested at GRIF and against SCDB examples.

To be included into 14.6.0 (https://github.com/quattor/template-library-core/issues/50)