Move wizard.scripts module to plugins, added hooks accordingly.

Move wizard.scripts module to plugins, added hooks accordingly.

Renamed the following functions:

    * scripts.fill_url -> deploy.web (now returns list of candidate
      urls and doesn't accept boolean old_style parameter)
    * scripts.get_disk_usage -> util.disk_usage (disk_usage now
      returns values in bytes, not kibibytes)

Scripts specific logic was refactored to scripts plugin.  Scripts
plugin got a setup.py file.

Created these plugin entry points:

    * wizard.deploy.web
    * wizard.user.quota

Added new functions:

    * user.quota (wizard.user module is new)

Fixed bugs:

    * git status on later versions of Git doesn't vary exit code;
      use diff-files and diff-index instead.

Restructured the deploy.Deployment.url code to run the generator of URLs
given to it by deploy.web.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>