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.

the power classes can raise multiple exceptions (timeout, foreman:exception etc), I would prefer if we only catch those we are expecting and not everything.
also, exception messages should be logged and possibly tracedump in debug logs too.