Add multi factor authentication method(s) for SSH and ncp admin panel

[theCalcaholic]

Description
Nowadays, MFA is considered a must-have for any kind of administrative access to web services, at least in a professional environment. Since suitable PAM modules exist, it should be easy to give users the option to enable MFA methods for SSH access as well as the admin panel (since it's based on system user authentication/pam as well).

I'm currently looking into this myself and if it's feasible with as little effort as I expect, I'll create a PR, provided that there are no objections to the idea.

Example
Some of the available pam libraries that could be used are:

• libpam-google-authenticator (which can not only be used with the official google-authenticator app, but also 3rd party apps)
• libpam-otpw
• libpam-yubico

However, I haven't evaluated any of these options in-depth yet.

[nachoparker]

I like the idea!