Allow for runners to execute more than one job per host
Description
Allow an auto-scaling runner to run multiple jobs in a single host. For example, in the current Docker Machine autoscaler, this would allow for more than one container to be hosted on a single ec2, thus reducing the number of ec2s that need to run concurrently. This could be controlled by a setting in the runner toml to allow different autoscaling runner instances to allow for different numbers of concurrent jobs in a single host. Default would be the current 1.