Add stable/xena job and drop stable/ussuri job

stable/ussuri has now been transitioned to EM (Extended Maintenance) [1].
So we can stop testing manila-tempest-plugin changes against that branch,
and introduce a stable/xena job.

Dropping the stable/ussuri job does not mean that manila-tempest-plugin
does not support running against that branch. Extended maintenance
branches (stable/ussuri, train and stein at the moment) still get
per-commit testing when changes are posted to the manila repo against
those branches.

Change-Id: I1e019ec913f8933d50afae6739352686725f4780
Signed-off-by: Goutham Pacha Ravi <gouthampravi@gmail.com>
3 files changed