Fix the broken use of the git module.

After this change, git will actually check out drush, instead of just
returning information about the repo. See
http://docs.ansible.com/git_module.html . TODO: Change the variable
name, or just get rid of it, and update the documentation.
Remove the drush_keep_updated variable.

It did not have the effect that the documentation claimed. See
http://docs.ansible.com/git_module.html .