juju#12473

## Documentation changes

Simply removes the stop between microk8s and subcommand to reflect the MicroK8s CLI. Both will work on Linux (so internal Juju calls to MicroK8s don't need to change), but only the space method will work on MacOS and Windows wrappers.

## Notes

This was rebased off 2.8 as requested in juju#12467

This fixes bootstrap failing with the following output:

Contacting Juju controller at 10.20.46.16 to verify accessibility...
ERROR unable to contact api server after 1 attempts: unable to connect to API: dial tcp 10.20.46.16:17070: connect: connection refused

If they do press Ctrl-C a second time, it'll show the "cleaning up"
message. Example output (Ctrl-C pressed 3 times):

^C
Ctrl-C pressed, stopping bootstrap and cleaning up resources
ERROR contacting controller (cancelled): unable to connect to API: dial tcp 10.99.244.80:17070: i/o timeout
^C
Ctrl-C pressed, cleaning up failed bootstrap
^C
Ctrl-C pressed, cleaning up failed bootstrap

Also move ctx.Done() check in BootstrapInstance to immediately after
non-nil err from StartInstance (simpler: not dependent on zones).

juju#12475

*NOTE: this is a refactored version of juju#12462: this version adds a `Context()` method to the existing `environs.BootstrapContext` interface so we're only passing a single "context" around, and to reduce churn.*

Bootstrap is a bit of a pain to stop/cancel half way through, even if it's obviously failing it keeps retrying stuff for a long time (eg: waitForAgentInitialization would take 10 hours before it gave up). Need to handle Ctrl-C and terminate/cleanup.

This PR adds a `context.Context` to the existing `BootstrapContext` interface, and checks `ctx.Done()` in retry loops and at various points (except in WaitSSH, where we can actually select on `ctx.Done()` properly). We're doing it in this relatively hacky way for now to avoid wiring context up through *everything*, e.g., providers, the old AWS library, etc.

In any case, Ctrl-C is now handled in several parts of the bootstrap:

* BootstrapInstance
* WaitSSH
* waitForAgentInitialization
* K8s Deploy() step

I've tested both a working bootstrap and a `^C`-interrupted bootstrap on lxd, k8s (minikube), and AWS clouds.

Sample output of interrupting `waitForAgentInitialization`:

```
# set up minikube as per https://discourse.charmhub.io/t/using-juju-with-minikube/3975/3
$ juju bootstrap kubernetes k8s
Creating Juju controller "k8s" on kubernetes
WARNING bootstrapping to an unknown kubernetes cluster should be used with option --config controller-service-type. See juju help bootstrap
Fetching Juju Dashboard 0.3.0
Creating k8s resources for controller "controller-k8s"
Downloading images
Starting controller pod
Bootstrap agent now started
Contacting Juju controller at 10.97.223.174 to verify accessibility...
 # hangs due to load balancer error
^C
Ctrl-C pressed, stopping bootstrap and cleaning up resources
 # max 3 second delay here
ERROR contacting controller (cancelled): dial tcp 10.97.223.174:17070: i/o timeout
 # during cleanup, I pressed Ctrl-C a couple more times just for kicks
^C
Ctrl-C pressed, stopping bootstrap and cleaning up resources

Ctrl-C pressed, cleaning up failed bootstrap
^C
Ctrl-C pressed, cleaning up failed bootstrap
```

For reference, juju@c946eff is the commit where Ctrl-C "handling" was introduced (basically to ignore it).

Conflicts:
cmd/juju/commands/bootstrap.go
provider/common/bootstrap_test.go
provider/rackspace/environ_test.go