Switch job definitions to use new "tempest_plugin" variable

This new variable was recently added to zuul in [1] to build
TEMPEST_PLUGINS list by combining plugin names with devstack
base path.

In old stable branches we still have to use old version with
TEMPEST_PLUGINS devstack option configured directly.

[1] https://review.openstack.org/#/c/639374/

Change-Id: I6bc2a3e752234d20890d64ceaefa0714cd05038f
Closes-Bug: #1822453
1 file changed