guests/redhat: Don't error if ifdown fails [GH-2614]

Further fix for [GH-2614].

Removed retrying of ifdown on the basis that in most cases the interface is probably missing and it adds an unnecessary 4 seconds to the setup time for the VM. The case where the interface is present and being removed by a config change would appear to be quite rare.

If someone can come up with a good way to distinguish both error cases I can to implement retrying in a more clever way.