Delay the stopping of instances when requested.

[lengau]

What needs to get done

Basically make #571 production-ready.

• Change Provider.launched_environment() to take an optional shutdown delay time (default to None using current behaviour, otherwise delay for the specified number of minutes).
• Change each Provider to be able to handle an already-running instance. (Perhaps setting and releasing a lock on that instance?)
• Change each Provider to run the shutdown command if the instance has a shutdown delay time set (even if that time is 0, in which case the shutdown should still occur asynchronously).

Why it needs to get done

During iteration on a (snap|charm|rock|whatever), each run includes starting up and shutting down an instance, which can add significantly to the time the run takes. A pre-warmed machine can avoid this.

NOTE: It will be up to the application to actually opt into using this feature, so it will be a backwards compatible change.

[sergiusens]

Multipass' stop has a timeout to delay the shutdown, we used to always use this with a timeout that would allow for iterations. Not sure why that is no where to be seen now

[lengau]

It looks like we still have code to use that, but the multipass provider doesn't use it.

LXD doesn't have the equivalent so we'll need to use my mentioned implementation there, but for multipass we can just expose that value.