Refactor filter_* methods as module functions

The various filter_* methods of the DiskCommon class don't operate on
the DiskCommon object instance itself, but always on the list of
objects passed in to it. This change does not modify those functions'
behaviors but simply refactors them to live at the module level.

Change-Id: Ic0f578d94aab65a524470278f10d1fdc8b1d6392
Signed-off-by: Peter Portante <peter.portante@redhat.com>
Reviewed-on: http://review.gluster.org/5138
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>