Docker module: add wait parameter

Add a `wait` parameter to the *docker* module.

When this parameter is specified, when starting or restarting a container, the module will block until the container has stopped. If the return value of the process is non-zero, the module will fail, using the process's output as the error message.

Example use case: initializing a database or running a database migration inside of a container. This task must complete before going to the next task in the playbook, and if it fails you need a meaningful error message.