templates/api-server: remove `url` from config

Modify "api-server" template and remove `url` from the generated
`server/config.json` file. That way loopback generates the URL
using the real HOST and PORT values.

This fixes a problem where environmental overrides were not honoured
in the URL printed at application start

    $ PORT=4000 node .
    Browse your REST API at http://localhost:3000/explorer
    Web server listening at: http://localhost:3000/

Related: https://github.com/strongloop/loopback/pull/659
Fixes #151 

/to @ritch @raymondfeng please review