README.rst: Add openSUSE Leap to list of supported distributions
README.rst: Add openSUSE Leap to list of supported distributions

SUSE support has been added a while ago and openSUSE Leap is being
tested in the OPNFV CI so add it to the list of supported
distributions.

Change-Id: Ie004ea88d02a35c57786b2249c2fbd94ac99ea7d

openstack/bifrost,openstack/bifrost