add -u / --mysql-user option to sandbox command

- This is used to specify a MySQL user name to add to
  the sandbox instance.  Previously this was always root@localhost
  but now a user specified $mysql_user@localhost user will be
  used.  If not specified, this defaults to 'root' and a
  root@localhost user will be added.  This is useful to avoid
  overwriting an existing user when bootstrapping from backup
  data.
Resolves issue #57
remove unused imports