CP-6167: Fix an issue reported by pylint
CP-6167: Fix an issue reported by pylint

Abstract class 'Attribute' referenced the undefined property
self.SYSFS_NODENAME. We rely on subclasses to always instantiate
this property but there is no guarantee. Better to use Abstract
Base Class module when we move to Python 2.6.

Signed-off-by: Zheng Li <fff69bf00b8ec5b6098a1768eaaaa75f40b8795d@eu.citrix.com>

xapi-project/sm,germanop/sm,pritha-srivastava/sm,chandrikas/sm,malcolmcrossley/sm,kostaslamda/sm,siddharthv/sm,malcolmcrossley/sm,pritha-srivastava/sm,BobBall/sm,martinjorge/sm,xapi-project/sm,djs55/sm,geosharath/sm,siddharthv/sm,BobBall/sm,robertbreker/sm,geosharath/sm,martinjorge/sm,malcolmcrossley/sm,germanop/sm,pritha-srivastava/sm,kostaslamda/sm,BobBall/sm,xapi-project/sm,djs55/sm,djs55/sm,chandrikas/sm,robertbreker/sm,kostaslamda/sm,geosharath/sm,robertbreker/sm,siddharthv/sm,chandrikas/sm,germanop/sm,martinjorge/sm