Removes the doc/high-availability-guide directory

- This document has moved to a new repo, openstack/ha-guide (https://git.openstack.org/cgit/openstack/ha-guide/)

Also, adjust files referencing it.

Change-Id: Ia6c0747fd00d0447ab26c61a2e24f5546bce2f0a
Merge "Removes the doc/high-availability-guide directory"