specify substituion args export directory in yaml file

[Fellfalla]

Problem

yaml file does not support substitution like "$(find map2gazebo)/models/map/meshes"

Solution

Load the yaml file in roslaunch with subst_value=true argument. reference: http://wiki.ros.org/roslaunch/XML/rosparam

e.g.

    <node pkg="yourpackage" type="yournode" name="nodename" output="screen">
      <rosparam file="path/to/yaml.yaml" command="load" subst_value="True"/>
    </node>