enable -o errexit

devstack should run under -o errexit to ensure that we fail early
when something has gone wrong, otherwise determination of the root
failure location is often quite challenging.

this clears all the normal use cases for devstack, there could be
tests which now die early, which we're going to have to check for
later.

Change-Id: Ibd828c4f4fd95a60d3918d3d7ae90e10649479ab
Merge "enable -o errexit"