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

Abstract class 'Link' referenced undefined property self.BASEDIR
and methods 'self._mklink' and 'self._equals'. We rely on subclasses
to always instantiate them 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>

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