Some small tidy-ups of the Chwala module

Summary:
The Chwala module is not really being use as a GenServer, so
don't bother with that overhead. The API is really where the
processes are launched (and so the async behavior comes from).

Also get the chwala api url from the application config.

Reviewers: WOPI Developers, machniak

Reviewed By: WOPI Developers, machniak

Differential Revision: https://git.kolab.org/D226