docker: Fix issues with stop operation

The docker RA's stop operation doesn't behave properly in some cases.

1. It returns a false success code in case of an error response from the daemon.
2. It fails at remove_container() if the container does not exist but another docker object of the same name does exist.

In case #1, the container_exists() function returns the same exit code
(1) if the container is not found (an expected error) or if there is an
error response from the docker daemon (an unexpected error). These types
of errors should be handled differently.

In case #2, the docker inspect calls do not limit their search to
containers. So if a non-container object is found with a matching name,
the RA attempts to remove a container by that name. Such a container may
not exist.

This patch fixes these issues as follows:

1. Match an error response in container_exists() against the string "No such container".
2. Add --type=container to the docker inspect calls to restrict the match.