Support syncing list of users & change 'pi' to 'users'

The term 'pi' is associated with a role in the PlanetLab system.  Not all users associated with a site have this role.  So, this patch changes use of 'pi' to the more general 'users'.

Second, to have better book keeping of M-Lab operators, this patch adds user syncing.  Rather than just adding users declared in user_list, syncing checks for users that need to be added as well as undeclared users that should be deleted from a site.  This will clean up the stray users listed by the site-audit scripts.