Allow for slow ramp up on creating new nodes

Something we've noticed is if you have a large job queue, lets say 100 and Jenkins is in Shutdown mode. When you deactivate shutdown mode openstack plugin will immediately launch as many nodes as it thinks it can start simultaneously. This often causes many nodes to start running the jobs at the same time too.

Edit: Another issue we noticed is since openstack-cloud-plugin uses the node count in openstack to determine if it's hit it's max yet, when this issue happens and max node count hasn't been achieved yet, openstack plugin occasionally ignores the max instances and launches more instances than it is actually allowed to launch.

This can cause a ddos against infrastructure from too many cloud nodes getting created at once, to Git servers being hammered by all the simultaneous git clones, to so many jobs hitting your download sites too quickly if they all need to download build artifacts, etc...

It would be nice if openstack cloud plugin was a little smarter in ramping up the number for nodes it spins up at a given time. Maybe allow this ramp up to be configurable eg. number_of_nodes and wait_time before starting new nodes.