Add configurable handler selection for Deployer

Added to solum-deployer the configuration value 'handler'
to select which one of the available modules in its
handlers directory to use as its endpoint. The result is
being able to configure exactly how images will be deployed.

At present we only have default.py, which uses Heat to deploy
a stack for the app, but in the future there may be other
handlers to use another process.

Since default.py was using Heat, I renamed it to heat.py and
added a new default.py mainly for contract enforcement.

Change-Id: I4c2aefc7d60e421c176bb8e854548486e9d800a3
Return assembly 'status' and 'application_uri'

On Assembly get, return the status and application_uri
fields. 

Change-Id: Ieb370c91f45f3edc48d7c4d2e1cf765a6e58f62b
Merge "Add configurable handler selection for Deployer"
Merge "Return assembly 'status' and 'application_uri'"