Add proposal for scheduled shutdown/pause

enhancements/proposed/20200424-scheduled-shutdown/20200424-scheduled-shutdown.md

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+# Scheduled shutdown and pause
+
+* First proposed: 2020-04-20
+* Authors: Thomas Stromberg (@tstromberg)
+  
+## Reviewer Priorities
+
+Please review this proposal with the following priorities:
+
+* Does this fit with minikube's [principles](https://minikube.sigs.k8s.io/docs/concepts/principles/)?
+* Are there other approaches to consider?
+* Could the implementation be made simpler?
+* Are there usability, reliability, or technical debt concerns?
+
+Please leave the above text in your proposal as instructions to the reader.
+
+## Summary
+
+Add the ability to schedule a future shutdown or pause event.
+
+This is useful for two sets of users:
+
+* command-lines which interact with a minikube on per-invocation basis. These command-line tools may not be aware of when the final invocation is, but would like low-latency between-commands
+* IDE's which start minikube on an as-needed basis. Not all IDE's have the ability to trigger exit hooks when closed.
+
+## Goals
+
+* The ability to schedule a pause or shutdown event
+* The ability to defer or update the scheduled event
+* "minikube start" transparently clears pending scheduled events
+
+## Non-Goals
+
+* Automatically idle detection. This is a related, but complimentary idea.
+
+## Design Details
+
+### Proposed interface:
+
+* `minikube pause --schedule 5m`
+* `minikube stop --schedule 5m`
+
+* Each scheduled pause would overwrite the previous scheduled event.
+* Each scheduled stop would overwrite the previous scheduled event.
+* Each call to `minikube start` would clear scheduled events
+
+As a `keep-alive` implementation, tools will repeat the command to reset the clock, and move the event 5 minutes into the future.
+
+### Implementation idea #1: host-based
+
+* If `--schedule` is used, the minikube command will daemonize, storing a pid in a well-known location, such as `$HOME/.minikube/profiles/<name>/scheduled_pause.pid`.
+* If the pid already exists, the previous process will be killed, cancelling the scheduled event.
+
+Advantages:
+
+* Able to re-use all of the existing `pause` and `stop` implementation within minikube.
+* Built-in handling for multiple architectures
+* Does not consume memory reserved for the VM
+
+Disadvantages:
+
+* Runs a background task on the host
+* Daemonization may require different handling on Windows
+
+### Implementation idea #2: guest-based
+
+minikube would connect to the control-plane via SSH, and run the equivalent of:
+
+```shell
+killall minikube-schedule
+sleep 300
+
+for node in $other-nodes; do
+  ssh $node halt
+done
+halt
+```
+
+Advantages:
+
+* Consistent execution environment
+
+Disadvantages:
+
+* Requires creation of a helper binary that runs within the VM
+* Untested: some drivers may not fully release resources if shutdown from inside the VM