Add instance type to message

[drewhemm]

When running multiple spot ASGs it is helpful to see in the notification which instance types are being terminated

[drewhemm]

Slack integration tested, results in messages like this:

[egeland]

Thanks for your contribution! 😄

Looks like you just need to sign your commits to get that check to pass - see the link.

[drewhemm]

Commit signed!