Use usermod(8) on OpenBSD to unbreak password management

[klemensn]

f1e77c2 "(PUP-3634) Hide password hash from process list for useradd"
introduced chpasswd -e which does not exist on OpenBSD, thus user
resources managing password would always fail:

Notice: Compiled catalog for atar in environment production in 0.02 seconds
rror: Could not set password on user[test]: No command chpasswd defined for provider openbsd
Error: /Stage[main]/Main/User[test]/password: change from [redacted] to [redacted] failed: Could not set password on user[test]: No command chpasswd defined for provider openbsd
Notice: Applied catalog in 0.01 seconds

Use https://man.openbsd.org/usermod.8#p instead:

Notice: Compiled catalog for atar in environment production in 0.01 seconds
Notice: /Stage[main]/Main/User[test]/password: changed [redacted] to [redacted]
Notice: Applied catalog in 0.21 seconds

password values now do show up briefly in the process list, but given
they must be encrypted in order to work, this does not seem critical.

[klemensn]

This is a cleaner version of what is already committed to OpenBSD's openvox package: https://github.com/openbsd/ports/blob/e3c8d71c07071f2171821b5b2bfb6f1702ce3466/sysutils/ruby-openvox/8/patches/patch-lib_puppet_provider_user_useradd_rb

[klemensn]

Linters are happy, tests used in CI seem broken and report failures entirely unrelated to this PR.