Implement ignore of mda if bit set by skipping r/w of metadata.

Implement ignore of mda if bit set by skipping r/w of metadata.

We implement ignore of an mda at label_read time by checking for
the ignore bit, and then skipping the reading of the vgname and
other information in the metadata.  This will have an effect similar
to a PV found with no mdas.  Thus, it will look like an orphan in the
cache until we scan the rest of the system and find a PV with
metadata, and the mda will not be on the vg->fid->metadata_areas
list so no read/writes will be done to the metadata area.

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>