Add support for pipeline failure notifications

[timbaileyjones]

mu doesn't currently have a way to specify a notification method for failed builds.

At the Re:invent 2017 talk DEV322, @clareliguori showed a failure notification feature.
It works by setting up a CloudWatch alert for failed builds, sending the event to a Lambda, which in turn forwards the event to SES (Simple Email Service) or a Slack channel. mu users will benefit from the shortened feedback loop.

The mu.yml would need to know the notification method, and the specifics about the message transport (SES email address, or Slack endpoint and API keys).

The first part of this is to notify when the build fails.

The next steps would be:

• Failure Notifications for any environment creation/update that failed (i.e. acceptance, production)
• Failure Notifications for any service deployment to those environments that failed (i.e. banana-service into acceptance, and banana-service into production)

Even notification failures to delete resources might be desirable (mu svc undeploy, mu env term, and mu pipeline term)