Support custom 'zeros' values, if available from the parameter server

Typical usage would be:

<node name="joint_state_publisher" pkg="joint_state_publisher" type="joint_state_publisher" >
    <rosparam command="load" file="$(find robot_description)/zeros.yaml" />
</node>

with zeros.yaml like that:

zeros: {joint1: 0.1, joint2: -0.2, ...}