Create temp directory on remote host owned by specified user (default: master's user)

This still needs rspec tests fixed but I thought I'd try to get feed back on two things while working on those:

* I did rename the method to look more like an existing method in the DSL, (create_remote_file) but that seems to be problematic due to the ambiguity about which method should be used, host.tmpdir or create_tmpdir_on(host)
* Instead of always assuming that the username used should be whatever the master's username is, I make that the default if no username is specified. I imagine this would be useful for projects that create additional users, for example classifier has a "classifier" user and group.