Fix F18 oz-install failures

F18 builds are unreliable (or for some people don't work at all),
this seems due to the following bug:

https://bugzilla.redhat.com/show_bug.cgi?id=906031

The simplest workaround is to use the netinst ISO instead of the DVD
so we pick up the newer curl package which fixes the problem

Fixes bug #1185810

Change-Id: I066356e8a780caa8ca2798644ec7594b0536005c