Zuul: Fix extra quotes when writing devstack localconf

Generate the correct TEMPEST_PLUGINS environment variable, moving to
the new variable tempest_plugins introduced with [0], to get around
devstack's behavior of quoting all arguments [1]. Maintain the old
syntax for releases earlier than stein.

[0] https://review.opendev.org/639374
[1] https://review.opendev.org/636078

Change-Id: I498c637da9dac7605851b2edbbfb302103c3d6a9
1 file changed