Add ability to define "active hours" for image groups

[mohsenrezaeithe]

Feature Request

Relates to #30

Use Case
Based on the same busy pool scenario, it'd be great to be able to shutdown all agents outside "active" or "busy" hours, which usually aligns with business hours. This would be a good money saver for those busy agent pools.

Functionality
A date picker for the Instances limit configuration on an image would describe when are the active/busy hours for the corresponding agent pool.