make default downtime duration configurable in cgi.cfg
|Target version:||Icinga 1.x - 1.5|
for both, flexible and fixed downtime, it's hardcoded within the sources (cmd.cgi)
fixed downtime uses the start_time+duration to calculate the end_time.
flexible downtime sets the duration to 7200 by default.
this should be marked as default_downtime_duration or similar.