Set concurrency of scenario tests to 3

In order to reduce the host load during the CI jobs execution,
the tempest concurrency for scenario jobs is reduced to 3 (from 4).
This should lighten the server load and reduce the occurrence of
random failures.

Closes-Bug: #1904858

Change-Id: I19ccb492506660dd7da56b48fe3ac9d0bc427f9d
1 file changed