Modify Docker set_user option to allow setting a remote user.
Now is disabled by setting this to the empty string instead of false. Add flexibility allows working around the fact that it is not evaluated properly for Pulsar by setting this to be the remote pulsar user :(. Longer term the default option should be the remote user on the worker node instead of the Galaxy user.

I don't believe this feature has been in stable so I am not preserving backward compatiblity.