Fixes #746 - Generate all the Host template when click on Build to avoid...

Generate all the Host template when click on Build to avoid errors during installation.
Includes moving of template_used to Managed model so it can be used elsewhere.

What are you rescuing here and in the exception in ```templates_status```? 

Maybe test what happens when the proxy is not available, or it doesn't have x or y feature and rescue for those here. For any other case raise a ```Foreman::Exception``` so we get an **[ERF number]**(http://projects.theforeman.org/projects/foreman/wiki/ErrorCodes) and we can track the issue. 

Same thing below for the template rendering.