Firstboot modules were installing into rhn dirs.

For RHEL6, we were installing firstboot modules into
the rhn client directories, because of interdependecies
between the rhn and rhsm modules. Now those are removed,
so we install to the normal firstboot module dir
(/usr/share/firstboot/modules).
Fix firstboot issues with new firstboot.

Since we now run last in firstboot, we were hitting
a case where the threads for the subscription api
calls were not finishing before we stop checking
the idle loop for events (which in this case, were
the idle source thread watcher methods).

Also handle no rhn-client-tool modules being installed,
and skip the steps that rely on them.
Fix firstboot module location on EL6.
Merge pull request #797 from candlepin/alikins/fix_firstboot

Alikins/fix firstboot