Fix the installer for EL7 and general logic changes

Pass yes to lvcreate so it overwrites LVs with signatures (which
shouldn't matter during the install anyway), and force wiping the
filesystem now that it's required.

Additionally, be smarter about whether to mount /liveos. Don't
try to mount any local filesystems at /liveos if we're booted
from a CD

Change-Id: I327fb0026d739361ad5c932a12cf3b536057be9a
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1095081
Signed-off-by: Ryan Barry <rbarry@redhat.com>