nuageinit: Accept plain text passwords

Per pw(8), when -H is set, the password should be supplied already
encrypted in a form suitable for writing directly to the password
database (passwd in cloud-init tems); -h provides a special interface by
which interactive scripts can set an account password using pw(8) in
plain text (plain_text_passwd in cloud-init terms).

The default user (freebsd) is defined with a plain_text_passwd
(freebsd), not with an encrypted one.

(cherry picked from commit 7b73ecfe648487c7706ac2b854dcf1435e60e4ca)