Set back to use Bionic for Ussuri, Train and Stein

With patch I2e37603b098880b778100613800496b01167b22f some jobs based on
tempest-multinode-full-py3 on stein, train and ussuri branches started
to to run on Focal, howerer those run on Bionic previously.
This patch sets back to use Bionic for those branches.

Change-Id: I9c30cf332a419946516cba776b8a3ecdb3cbd3c2
1 file changed