Add support for minimum num of instances as hot standbys

[mohsenrezaeithe]

Feature Request

Use case
Add ability to define a minimum number of instances for pools that are busy, that are known to have intermittent quiet periods that exceed the Terminate instance idle time configuration in a Cloud Profile. In our scenario, we're ok with a set number of instances that are kept alive and idle to avoid the time a build needs to wait for at least one agent to spin up and become available.

Functionality
For a first version, it'd be useful to add another configuration to the Instances limit under an image configuration to allow for configuring a minimum number of instances for the image group: