Support Docker container labels

[FroggyFlox]

Many Docker containers now make use of containers labels and allowing the user to specify labels would be much helpful in establishing multi-containers and even multi-Rock-ons environment.

As there isn't--to the best of my knowledge--support for container labels to docker update, this would need to be done at docker run time, which in Rockstor could be done either at the RockOn definition stage (using the options object) or using the update function. As labels would be specific to each user's environment, the latter is greatly preferred.

I thus suggest to implement an "Add label" feature from the Rock-On Settings page, similar to the existing "Add Storage" feature. This would also need to support adding several labels at once as multiple labels are usually needed to setup a custom multi-container multi-RockOn environment.

I'm currently finishing a related branch and will prepare the PR quite shortly.