The `rally verify start` command doesn't allow us to specify
concurrency for Tempest tests. The tests always are run in parallel
mode and use concurrency equal to the count of CPU cores. However,
users may want to use specific concurrency for running tests based
on their deployments and available resources.
This patch adds possibility to specify desired concurrency for tests.
Closes-Bug: #1524968
Change-Id: I3ba2c987c739b13bb03005b1988ee6bd01e8a08d
331b9ac8d5