system: Fix is_el(…) semantics

Previously the command raised and error, but that is rather cumbersome.
Now the semantics of the command are that it will only be true if it's
el* and the specified version.

Change-Id: Ia97bac4bafca347596e61af71e651821cca2a4a5
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>