Add mda_copies to VG structures and initialization.

Add mda_copies to VG structures and initialization.

Add a field to struct volume_group to later implement metadata
balancing:
- mda_copies: target # of non-ignored mdas in the VG; default 0 (do
not control pv 'ignore mdas' bit.

This patch just adds the parameter to the structures with the default
values but does not modify any commands.  Should be no functional change.

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