Support expiring acknowledgements for Naemon

[roelvanmeer]

Now that Naemon has support for expiring acks (naemon/naemon-core#302), it would be nice if Thruk would support this.

I can write up a change, but I'd like to know how you'd prefer this. Currently the enable_icinga_features and enable_shinken_features variables are used to enable this, but enable_naemon_features doesn't exist. Do you want to create that, or can we just allow this feature to be enabled regardless of the backend? I didn't check but I can't imagine Nagios doesn't have this feature as well.

[sni]

I have prepared a patch already, no worries. Will push it as soon as the change it merged.

[sni]

expire acks are now enabled by default with bf9afef
But they can be disabled from the config if necessary.