datamodel: renaming 'server' section

This section is basically a summary of things that do not fit elsewhere, so the name of this section may be confusing.

examples of the new name: general, management, ...

Or maybe try to split the section.

Current server section

server:
  id:
  hostname:
  nsid:
  workers:
  use-cache-gc:
  backend:
  watchdog:
  rundir:
  management:
  webmgmt: