F checkcommand static template

Updated to allow you to configure a check command via a 'plain' file with custom erb template or source file.

Also update the parameter checks to only validate some if using the default .erb template
Would you be able to make this default to an empty hash and change the accompanying code in the ERB template?

The `EventCommand` defined type would be a good model to follow:

https://github.com/Icinga/puppet-icinga2/blob/master/manifests/object/eventcommand.pp

https://github.com/Icinga/puppet-icinga2/blob/master/manifests/object/eventcommand.pp