Fix : #1269 #1271

Hi,

I've created a new file in order to factorize code a little regarding Exernal Command.

This pull request has to be merged with shinken-monitoring/mod-livestatus#34

I will merge  it in 1 week. Feel free to give a feedback before :)
I think we should not put in retention such thing. It's not a simple parameter, it's a full object with links and all. If it was in retention it was an error, but we should remove such complex objects from retention. Even ifthe nagios manage it, we should not. Modifying a check_command or a command in running phase is a very, very bad idea, and will lock for future enhancement (like X checks commands or things like this).

I'll remove this.