[builder] Rework implementation of add_default_interface

The new implementation only adds the default service and datastream
entry if the sensor | actuator does not get already some service or
datastream configured for this specific middleware. Moreover, it now
handles properly the sensors and actuators added after the call of
add_default_interface.

It should fix #399 (but people interested by the features must test to
see if it matches their expected semantics).
[builder] Update deprecated overlay function in pr2 description
[ros] Fix double self-reference
[human] Reload scene when human is used
[cmake] Force the python executable when lauching sphinx-build

Fix some build issue when both python2-sphinx and python3-sphinx are
installed. Reported by M. Bulk from robotpkg.