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
Merge "Add RHEL 7 cloud image link to guide"