setup_spawnhost_coredump script should notify user via Slack when it has finished running

The wall command is great for having the banner to tell me when I've ssh'd in too early. Having the notification come through Slack when everything is finished means I would only need to look once though.

Proposed Slack notification text:

Linux

The setup_spawnhost_coredump script has finished setting things up. Please run

ssh ec2-user@ec2-xx-yy-zz-ww.compute-1.amazonaws.com

to log in.

Windows

Using Cygwin for remote debugging on Windows is strongly discouraged. We should steer Server engineers to use Windows Remote Desktop instead.

The setup_spawnhost_coredump script has finished setting things up. Please use Windows Remote Desktop with

1. PC name: ec2-xx-yy-zz-ww.compute-1.amazonaws.com
2. User account: Administrator
3. The RDP password configured under the edit dialog at https://spruce.mongodb.com/spawn/host

to log in.