T6818: Add argument to vyos_config for controlling password filtering (#466)

This commit adds the argument allow_password_change in order to
control whether any configuration lines which would make changes to user
passwords should be filtered out or not.

Co-authored-by: Daniil Baturin <daniil@baturin.org>
Co-authored-by: omnom62 <75066712+omnom62@users.noreply.github.com>