ansible2gdeploy - enhance hosts replacement functionality

[dahorak]

It would be useful if it will be possible to limit or specify number of "used" hosts. I'm thinking about following two specifications:

• a precise number,
• maximum available multiple.

[mbukatov]

I would suggest use the same approach as we do for devices. That is, include some example host name in the hosts section, and then let ansible2gdeploy to replace it with actual names based on inventory file.

That way, the number of machines would be specified exactly (nicer way to say hard coded) in the gdeploy file. And given the fact that we require specific number of hosts in the name and comment of the gdeploy file, I think that this approach is ok.