nuageinit: implement chpasswd

Add support for chpasswd, with all possible syntaxes, including
deprecated one: chpasswd.list as a list or as a multiline string
as some providers are still only providing this deprecated form

Approved by: re (cperciva)
Sponsored by: OVHCloud
MFC After: 1 week
Reviewed by: kevans, jlduran
Differential Revision: https://reviews.freebsd.org/D50021

(cherry picked from commit c201a1198ad70e7d096ee32c364d539eed2dfec4)
(cherry picked from commit 6c912470030ba958f2e41a00b44f6430919b1389)