Purge all deleted rows by glance manage in one step.

When user wants to purge all deleted rows he/she uses "glance-manage db purge" and "glance-manage purge_images_table". An user never knows how many deleted rows are still in a table. So he needs to launch the script many times until the script reports that 0 rows were deleted in every table. It's inconvenient.
We need to add command-line option "--purge_all" to purge all deleted rows in one step.