WIP: add generator modules

[KiaraGrouwstra]

adjusts clan-core's generator modules for use outside of clan.

as part of this, i have:

• made them easy to import, while gating their functionality behind enable flags
• had modules presuming a single user (user-password, sshd/shared) support multiple users
• better facilitated unattended scenarios, adding a vars.unattended flag as a global flag for modules' individual prompt flags

status:

• better consider namespace
• testing

closes #9.