Add package management and miscellaneous option snippets (repo, rescue, skipx, xconfig)

[sfloess]

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

Scope

Add option snippets for remaining kickstart commands not covered by other sub-issues:

• repo - Configure additional yum repositories for installation (repo --name= --baseurl= | --mirrorlist= [--cost= --excludepkgs= --includepkgs= --proxy=])
• rescue - Boot into a rescue environment instead of installing
• skipx - Do not configure X Window System on the installed system
• xconfig - Configure the X Window System (xconfig [--defaultdesktop= --startxonboot])

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