arresting bootstrap teardown

[roger peppe]

If anyone, like me, has been frustrated when debugging
bootstrap failures and having the bootstrap
machine torn down immediately on failure,
a quick and relatively easy workaround for that
is to "kill -STOP" the juju bootstrap process
while it's doing the ssh commands.

You'll continue to see the ssh commands execute,
but the parent process will stop when they finish,
allowing you time to ssh into the bootstrap machine
and inspect it.

kill -CONT to allow the process to complete its cleanup.