Add user, group, and authentication option snippets (user, group, sshpw, realm, eula)

[sfloess]

Part of #15: Include all option snippets as found in the kickstart guide.

Scope

Add option snippets for user management and authentication kickstart commands. Currently only auth and rootpw exist. The following are missing:

• user - Create a new user (user --name= [--groups= --homedir= --password= --iscrypted --uid= --gecos= --shell=])
• group - Create a new user group (group --name= [--gid=])
• sshpw - Set a password for SSH access during installation (for remote monitoring/interaction)
• realm - Join an Active Directory or IPA domain (realm join [--server-software= --client-software=] domain)
• eula - Accept the End User License Agreement without user interaction (eula --agreed)

Reference

RHEL 7 Kickstart Syntax Reference - Section 27.3

Acceptance Criteria

• Each command has a corresponding snippet file under snippets/options/
• Snippets follow the existing pattern/style used by other option snippets in the repo
• Key options for each command are parameterized via cobbler variables