Allow for maximum time to keep a container alive

[ketan]

Currently the plugin will aggressively delete the container as soon as it sees that the agent is idle. Would like to see the ability to terminate the container only if it remains unused for X amount of period, or just put a soft and hard upper bound on how long a container should remain alive.

Soft upper bound being a point where the agent will be disabled. Hard upper bound being the point where the container will be terminated (even if it is running a build). This would be useful in cases where the agent or build that is running on the container hangs up.