Add note about build_timeout

Compute's build_timeout applies to other services that don't define that
value. This patch adds a comment to make that explicit.

Change-Id: I8bbe7a654a548317d0c66fa24506eff7424cf9ef
Closes-Bug: #1394519
Merge "Add note about build_timeout"
Merge branch 'master' of https://github.com/openstack/tempest