Added --limit option to teuthology-suite.

Use --limit to limit the number of jobs being scheduled during
teuthology-suite. Also can be used with schedule_suite.sh via the
10th argument.

Signed-off-by: Sandon Van Ness <sandon@inktank.com>
Signed-off-by: Yuri Weinstein <yuri.weinstein@inktank.com>