Add lvm_vg_get_tags(), lvm_vg_add_tag(), and lvm_vg_remove_tag().

Add lvm_vg_get_tags(), lvm_vg_add_tag(), and lvm_vg_remove_tag().

Add lvm2app functions to manage VG tags.
For lvm_vg_get_tags(), we return a list of tags, similar to other
functions that return lists.  An empty list is returned if there
are no VG tags.  NULL is returned if there is a problem obtaining
the list of tags.

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