Add screen to describe and skip registration in Fedora/EL7 firstboot.

Uses the informational screen from rhn-client-tools, but only displays
it on the operating systems where we run firstboot without that package.
(Fedora, and RHEL 7+)
Hook up the 'why register' dialog from old rhn-client-tools.

Slightly different style of display to keep this in front of firstboot
itself, and prevent interaction with firstboot until it is closed.
Fork firstboot behaviour for RHEL 6.

Head straight to choose server screen if on RHEL 6, where
rhn-client-tools screens already displayed the registration info.
Merge pull request #811 from candlepin/dgoodwin/skip-firstboot

Allow skipping registration in RHEL 7 / Fedora 19 firstboot.