ec2-api job: limit tempest workers for testing

because ec2-api functional tests are very aggressive then
four workers can exceed quotas. but two workers should work well
as we saw in the past.

Change-Id: I1b1b735c5f3eb5af308d7de55e4de89ed0161a13

jenkins/jobs/ec2-api.yaml

@@ -60,6 +60,7 @@
           export PYTHONUNBUFFERED=true
           export DEVSTACK_GATE_TIMEOUT=120
           export DEVSTACK_GATE_TEMPEST=1
+          export TEMPEST_CONCURRENCY=2
           export BRANCH_OVERRIDE={branch-override}
           if [ "$BRANCH_OVERRIDE" != "default" ] ; then
               export OVERRIDE_ZUUL_BRANCH=$BRANCH_OVERRIDE