Remove CentOS 7 as supported environment
Remove CentOS 7 as supported environment

We do not support CentOS 7 on master branch due to no Python 3 or
ongoing eco-system (i.e. RDO) support; see
Id9ef507dd6f4226d65c6ed3043666b0aa6a3bd1c.

Change-Id: If98581708568e7a8d15e6edc588a008df0cac0fb

openstack-dev/devstack,nawawi/openstack,openstack-dev/devstack,nawawi/openstack