[sshd] Disable password authentication.

[sshd] Disable password authentication.

Client can authenticate with `sshd` through one of following
authentication methods (corresponding sshd_config option in brackets):
- host-based (`HostbasedAuthentication`),
- public key (`PubkeyAuthentication`),
- challenge-response (`ChallengeResponseAuthentication`),
- password (`PasswordAuthentication`).

By default, only `PubkeyAuthentication` and `PasswordAuthentication`
are enabled. This commit disables `PasswordAuthentication`. Users
can now login only using public key authentication.