Add a Debian init script for salt-api

It seems that salt-api doesn't have an init-script on Debian, which may be the cause for this salt-bootstrap issue:

https://github.com/saltstack/salt-bootstrap/pull/442

Adding this file to my repo and running salt-boostrap with that one (using -g) enabled the installation to proceed, and I was able to launch salt-api (after installing Cherrypy using pip and adding Cherrypy configuration).