Xacro should not use plain 'include' tags but only namespaced ones.

This is an unfortunate conflict because it might break some people's launch files, but xacro currently is not obeying its namespace as it says it does in its documentation. Gazebo's <gazebo> extension has its own <include> that is only used within its <gazebo> element, but Xacro is improperly trying to parse these as Xacro includes.

More details on Gazebo's issue tracker:
https://bitbucket.org/osrf/gazebo/issue/730/xacro-not-compatible-with-urdfs-sdfs

I'm only proposing we make this change in Hydro...

Reviewers
@jonbinney @jbohren  @nkoenig @hsu