Add command to benchmark either a PHPUnit test case w/ custom Piwik related code or Piwik URL

As title, new command `tests:benchmark` will load a fixture into the DB (specified through command line arguments and can be your own custom file) and then benchmark either a phpunit test case or URL. The phpunit test case can be your own custom test case (so you can benchmark any bit of Piwik code). Benchmarking can be done on AWS as well as locally.

Includes some refactorings to CliMulti/RequestCommand/RunTestsOnAws/GenerateTravisYmlFile.

CC @mattab @mnapoli @tsteur Reviews welcome.