Fix: change slack success and fail messages

[Aquive]

trafficstars

• [x] Bug fix (Slack Messages)
• [ ] New feature?
• [ ] BC breaks?
• [ ] Tests added?
• [ ] Docs added?

Slack message was as follows:

MyProject
Deploy to *HEAD* successful

Where something like below was intended.

MyProject
Deploy to *myproject.com* successful

I changed it to:

MyProject
Deploy of `HEAD` to *myproject.com* successful

Same for failed message.

[antonmedv]

This will work strangely on multiple hosts deploys. I think we need some sort of notion of "stages".