Support for building a Fedora 18 iso

This initial set of changes produces an iso. The iso
boots and runs the installer.

/tftpboot/redhat_dvd/extra is missing on the installed
system. This needs further investigation.

Includes contributions from cwolfe.
Download Fedora 18 ISO if missing
Remove unused sledgehammer bits
Merge pull request #1748 from rwsu/master

https://github.com/crowbar/crowbar/pull/1748

Merged by devtool for VictorLowther