Use wipefs to clear old signatures before creating new file systems (#688882)

Previously the function erase_filesystem_signatures() was used to clear
file system signatures when a new partition was created and when an
existing partition was formatted with a file system.  However this was
only available with libparted <= 2.4 and then only for the file systems
which libparted supports.

Having multiple different file system signatures on a partition leads to
misidentification of file system.  For example creating a nilfs2 over
the top of a fat32 file system is detected as a fat32, not nilfs2.  This
shows that old file system signatures must be cleared before a new file
system is created.

Fix by always using "wipefs -a /dev/PARTITION" command to clear all old
file system signatures rather than libparted API calls.  Failure from
wipefs is only considered a warning so doesn't fail the file system
creation.  (This doesn't yet fully meet the "MUST be cleared"
requirement above.  Will be fully met later in this patchset).  Output
from the wipefs command is displayed as a new sub-step which looks like
this:

    v Format /dev/sda7 as xfs                            00:00:05
      > calibrate /dev/sda14                             00:00:01
      v clear old file system signatures in /dev/sda7    00:00:01  [NEW]
        > wipefs -a /dev/sda7                                      [NEW]
      > set partition type on /dev/sda7                  00:00:02
      v create new xfs file system                       00:00:01
        > mkfs.xfs -f -L "" /dev/sda7

Also signatures are only cleared immediately before a new file system is
written and not when an unformatted partition is created.  This allows
recovery from accidental partition deletion by re-creating the deleted
partition as unformatted.

Bug #688882 - Improve clearing of file system signatures