Cherry picked from https://review.openstack.org/#/c/15983/

Fix bug 1061665, --force_hosts hint for launching instance updates

--force_hosts no longer available in Folsom, have to indicate
an availability zone instead.

Patchset attempts to explain how to get the availability zone
name that you want. Please review.

Patchset addresses comments about zone versus host, adding
new section rather than replacing host section completely.

Change-Id: Ifea0df3dc950c165718fa160bc08f1b1487c3f82