Updates to use 1.7.2 to build openstack-install.

Patchset resolves the two broken links at:
<link linkend="ch_cells">Cells</link>
<link linkend="configuring-migrations">live migration</link>
by converting them to link xlink:href and doing a relative
link to the Compute Admin manual HTML file that is output.
This way, the build works and the user can still click to
get the needed information.

Change-Id: I44dfc5965e78568c172c95b86f71324fb3f2ff7c
Merge "Updates to use 1.7.2 to build openstack-install."