Setting Tempest run concurrency to 4 for a few jobs

We recently changed the default concurrency to the higher
value (number of cpu -2) which end up 6 in upstream CI.
Higher concurrency means high parallel requests to services
and can cause more oom issues. To avoid the oom issue, setting
the concurrency to 4 in a few of the jobs which run more
parallel tests

Related-Bug: #2004780
Change-Id: Ifa2ac35453e17ca01378ebebb310a4719b704fef
1 file changed