sysutils/py-salt: Make salt.modules.pw_user.get_loginclass return string
rather than dict.

This fixes salt.modules.pw_user.chloginclass which is used for FreeBSD
and expects a string to be returned. This was therefore returning False
even after successfully changing the loginclass.

This also fixes salt.states.user.present on FreeBSD when loginclass is
set.

PR: 218694
Approved by: Christer Edwards <christer.edwards@gmail.com> (maintainer)