Signal AWS when applications are deployed

[jbardin]

We currently can't do rolling upgrades of the galaxy image because the apps often take longer to deploy than the instances.

Galaxy needs to lookup all applications that should be running on a host, and notify the ASG when deployment is complete.

The ASG can have an UpdatePolicy, or the stack can contain a CreationPolicy to define when an instance is ready. Galaxy can use the API or the cfn-signal script for notification.

[jbardin]

Since we've removed the cloudformation dependency from galaxy, this should be implemented in such a way that it's not coupled to AWS.

A callback command when the host is up should suffice.