[EC2] Adds retry on host not available, add some useful debugging.

- debugging, to get the whole message
- add backtrace to compute resource failure msg
- add debug logging for Fog errors on create

fixes #1351 New smart variable and match cannot be entered at same time.

smart vars looks sharper.

fixes #1572

fixed useless warning

fix error in validation for new compute hosts

when libvirt is not enabled, compute resource provider name is incorrect.

fixes #1669 - epel is now a snippet.

This adds another method to install the epel repo, as a snippet,
allowing users to customly change the url, in case that it doesn't fit
their needs or if the url changes again.

to use, simple use:

<%= snippet "epel" %>

fixes #1575 error 404 on dhcp proxy

fixes #1605 - typo thier