Incidentally, fixes an acceptance test (which would have caught this
bug) not running. Its step regex was "They are correctly selected
after I reload the page$", which happens to be matched by "I reload
the page$", another step definition. Lettuce seems to arbitrarily pick
between definitions if they both match a step, so it's good to ensure
that each step isn't a sub-regexes of another.
Instead, we use XModule field default values when creating an empty
XModule. Driven by this use case, we also allow for XModules to be
created in memory without being persisted to the database at all. This
necessitates a change to the Modulestore api, replacing clone_item with
create_draft and save_xmodule.
