Refactor teuthology.beanstalk

This architecture will make it easier to add new functionality.

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Add --runs, to print only run names

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Calculate a timeout to use based on queue size

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>