ipa-kdb: cache KDC hostname on startup
    
    We need KDC hostname for several purposes:
    - short-circuit detection of principals on the same server as KDC
    - generating NetBIOS name
    
    Make sure we cache hostname information on startup and use it
    instead of detecting the hostname in run-time. This will miss the
    case that KDC hostname got changed but such cases are not supported
    anyway without restarting KDC and making changes to principals.