Add a solum-api command

There are a couple of reviews up for deciding on the REST API
infrastructure.  Regardless which is picked, they will need a solum-api
command that serves up the API.  This commit puts those base pieces in
place.

Part of blueprint rest-api-base

Change-Id: Ie7d22d3ece0e81eca7959ad2d3ba5d756264d797