Replace Hipchat with Slack for the deploy notifications.

[benlk]

[benlk]

Idea/question: We should ask if anyone else is using our deploy tools and if they're using them with HipChat notifications. And if they are, we could leave the HipChat notifications in the tool, but also add Slack.

Otherwise, just cut HipChat and add Slack.

[benlk]

Sample code: https://github.com/os/slacker/blob/master/examples/post.py

Message payload information: https://api.slack.com/incoming-webhooks

Put Slack API tokens in an env var. Channels would need to be configured per-umbrella, so that would go in the sample fabfile, perhaps defaulting to #general in the sample and to #devops in INN/umbrella-boilerplate. The app should not even attempt to send a notification (should not segfault) if any of the requisite variables are unset, unlike the current Hipchat integration.