Draft support for HTCondor clusters
htcondor.yml: Use the debconf mechanism for configuration.
HTCondor: make value of DebConf template `condor/daemons` depend on master/worker
HTCondor: Adjust DebConf template `condor/daemons`.

Apparently the correct configuration is this one:

* Submit only node:            DAEMON_LIST = MASTER, SCHEDD
* Submit & exec node:          DAEMON_LIST = MASTER, SCHEDD, STARTD
* Master (no submit, no exec): DAEMON_LIST = MASTER, COLLECTOR, NEGOTIATOR