Add substitution args parsing to roslaunch yaml loaded from file

[DSsoto]

@jbohren's #218 added resolving args in rosparam if the yaml text is inlined inside of the launch file.

However I have repeatedly found myself missing this feature when loading parameters from a external yaml file using rosparam's file attribute. Here is a minimal reproducible example of the issue.

config.yaml:

# This arg will not get substituted corrctly
subbed_pkg_from_file : $(find roscpp)

param_loader.launch:

<launch>
  <rosparam subst_value="true"> subbed_pkg : $(find roscpp) </rosparam>
  <rosparam subst_value="true" file="/home/santiago/config.yaml" />
</launch>

Output of roslaunch param_loader.launch --dump-params:

{/subbed_pkg: /opt/ros/kinetic/share/roscpp, /subbed_pkg_from_file: $(find roscpp)}

It would be very useful to actually have subbed_pkg_from_file resolve to the package path when subst_value is set to true.

[dirk-thomas]

The maintainers will likely not have time to implement this in the foreseeable future. Please consider to contribute a pull request to implement this feature.

[dirk-thomas]

Implemented in #1354.