Allow to control creation of new elastic agents on a plan by plan level

It would be really helpful if in addition to a global elastic Bamboo configuration where I can how many plans need to sit in the queue for how long to start a new agent we could tell that certain plans are OK to be sitting in the queue. Some of the build plans we run are in a way time sensitive and I am all for spinning off a new agent to handle them if a congestion occurs. Other plans are much less sensitive to wait times and can totally wait while the existing agent(s) free up to run them.

This is for the On Demand version