[Feature] Ability to patch 1 app easily

[aelkugia]

Customer mentioned it would be nice to have the ability to patch only 1 app easily. Today the process includes the following:

• Loading a new docker image into a node’s daemon inside gravity
• Push to gravity registry
• Pull from every node
• Deploys are configured to pull image IfNotPresent so the image is in every node’s local daemon
• When servers are restarted, a new gravity master unpacks the applications and the pushed image is lost, only relying on the one in the local daemons