layer.conf: define ROSDISTRO within the meta-ros layer

Define a ROSDISTRO variable to record the current ROS distribution
code name for use within ROS-related recipes.
classes: locate packages in /opt/ros/${ROSDISTRO}

Following standard convention, place ROS/Catkin packages under the
prefix /opt/ros/${ROSDISTRO} (exported as ${ros_prefix}) rather than in
the usual ${prefix}.
README.md: update to reflect new ROS package prefix

Update the setup instructions to reflect the use of ${ros_prefix}.
ros-realtime-tests: updating to 0.1.3

This version fixes build problems with a ${prefix} other than /usr.
tf2-py: suppress rpath usage for Python extension libraries
Merge pull request #286 from housel/relocate-to-opt-ros-rosdistro

relocate ROS packages to /opt/ros/${ROSDISTRO} (resolves #214)