Add support to RHEL 7

RHEL 7 is the next Red Hat Enterprise distro version and because of a difference
in the distro name in Volume Descriptor Table on it's ISO file, Kimchi is not
recognizing as a RHEL distro.
This patch adds the correct regex to add support to RHEL 7

Signed-off-by: Adriano Botega <abotega@linux.vnet.ibm.com>