install: Use correct arguments for wipefs

wipefs on el7 takes different arguments than wipefs on el6.
A recent patch added arguments (-f) to wipefs which ain't supported on
el6, and thus broke the installation.

Now different arguments are used on el6 and el7.

Change-Id: Ie28f45bda046f386933767e0998a3328ef79a915
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1150377
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>