The "Developing Modules" documentation states:
Include a minimum of dependencies if possible. If there are
dependencies, document them at the top of the module file, and have
the module raise JSON error messages when the import fails.
When docker_service runs on a remote host without PyYAML it crashes with
ImportError.
This patch raises a JSON error message when import fails, but only if
the PyYAML module is actually used. It's only needed when the
"definition" parameter is given.
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
