Fix mysql element on centos7
Fix mysql element on centos7

The distribution element for CentOS (there is only one) is called
centos7, so fix the check for the distribution name to make this
element behave when building a CentOS image.

Change-Id: Id329228c558e7adbb7d285503dde992df4495510

rdo-management/tripleo-image-elements,openstack/tripleo-image-elements,rdo-management/tripleo-image-elements,radez/tripleo-image-elements,radez/tripleo-image-elements,openstack/tripleo-image-elements