Forced serial execution of tempest test suit

Forced serial execution of tempest test suit

"-s" option is added for run_tests.sh in order to force serial
execution of test cases in Functest test suites. This option
affects currently Tempest test suite.

Using of this option will reduce number of consumed floating IP
addresses and should resolve possible concurrency problems
of testr tool. On the other hand, it will increase the total
execution time of test cases.

Change-Id: Ib685db5faa94ed498ff9a0ec6587786b60f0c73d
Signed-off-by: Viktor Tikkanen <viktor.tikkanen@nokia.com>