Add RHEL 7 cloud image link to guide
Add RHEL 7 cloud image link to guide

Added RHEL 7 cloud image link to guide, removed boot.iso image to
avoid confusion as it is a different type of image to the rest of
those discussed in this guide and further instructions on its use
are likely required.

Change-Id: Ib81e9f13f69a389a4db7b1d7d191e10e2729e463

AlekhyaMallina-Vedams/openstack-manuals,Akasurde/openstack-manuals,kairoaraujo/openstack-manuals,saeki-masaki/openstack-manuals,potsmaster/openstack-manuals,potsmaster/openstack-manuals,gongysh2004/openstack-manual,saeki-masaki/openstack-manuals,kairoaraujo/openstack-manuals,openstack/openstack-manuals,gongysh2004/openstack-manual,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,Akasurde/openstack-manuals,openstack/openstack-manuals