Split clear providers

* clear_providers() is now split into clear_infra_providers() and
  clear_cloud_providers.
* The has_no_providers fixture has been joined by has_no_cloud_providers
  and has_no_infra_providers.
* Tests in the cloud and infrastructure directories of the new cfme code
  base have been updated to clear their respective provider types.

The rationale behind this is to a) reduce the risk of provider clearing
the wrong provider whilst developing and b) to speed up the fixture, as
it has to visit pages multiple times. Also, the delete validation is now
only triggered if there are providers to delete.