Set link name from rosparam for test_stability.cpp

I want to use test_stability.cpp for my robot, HRP-2.
But now, test_stability.cpp has no option for setting the name of support link.

In this pull request, test_stability.cpp set link name from rosparam.

"test_stability/root_link_name" for root_link_name_
"test_stability/rfoot_link_name" for rfoot_link_name_
"test_stability/lfoot_link_name" for lfoot_link_name_
"test_stability/rfoot_mesh_link_name" for rfoot_mesh_link_name_
 
of cause, if there is no rosparam, these values are set as the original values.

Thanks.