system.py: add centos for is_el validation

is_el() should work for based RHEL distro. Commit 3d890b3
fails in case cannot find el distro. This patch adds
the validation for centos.

Change-Id: I2e636183e468fe35c04c3c950eefceacf0e4b1b7
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>