Skip Swarming tests if MySQL is not installed

add a parameter --runSwarmingTests to run_tests.sh , run_tests.py and swarming_test.py 
which is unset by default. Swarming tests are executed only if this parameter is passed. 
Then make Travis execute run_tests.sh with this flag, so we get whole tests coveradge. 

Motivation for this is that accessing MySQL on user machines may be problematic (and we dont require the DB as a dependency)