Changed key name to check for provider_region instead of hostname

Changed key name to check for provider_region instead of hostname, EC2 specific code was changed to use provider_region column instead of hostname. Wrong key name in the check was causing validate button to not be enabled when adding EC2 provider.

https://bugzilla.redhat.com/show_bug.cgi?id=1146776
https://bugzilla.redhat.com/show_bug.cgi?id=1147075

@dclarizio please review/test. This broke with changes for https://bugzilla.redhat.com/show_bug.cgi?id=1144523