Use python3 specific rosdep key for nose.

[nuclearsandwich]

Without this there are downstream failures like this one.

11:06:17 	cmake .. -DCMAKE_INSTALL_PREFIX=/usr -DCMAKE_VERBOSE_MAKEFILE=ON -DCMAKE_BUILD_TYPE=None -DCMAKE_INSTALL_SYSCONFDIR=/etc -DCMAKE_INSTALL_LOCALSTATEDIR=/var -DCMAKE_INSTALL_PREFIX=/opt/ros/r2b2 -DAMENT_PREFIX_PATH=/opt/ros/r2b2
11:06:17 -- Found ament_cmake: 0.0.0 (/opt/ros/r2b2/share/ament_cmake/cmake)
11:06:17 -- Found PythonInterp: /usr/bin/python3 (found suitable version "3.5.2", minimum required is "3") 
11:06:17 -- Using PYTHON_EXECUTABLE: /usr/bin/python3
11:06:18 -- Found ament_lint_auto: 0.0.0 (/opt/ros/r2b2/share/ament_lint_auto/cmake)
11:06:18 -- Added test 'copyright' to check for copyright in CMake / C / C++ / Python code
11:06:18 -- Added test 'flake8' to check Python code syntax and style conventions
11:06:18 -- Added test 'lint_cmake' to check CMake code style
11:06:18 -- Added test 'pep257' to check Python code against some of the style conventions in PEP 257
11:06:18 Traceback (most recent call last):
11:06:18   File "/usr/bin/nosetests", line 5, in <module>
11:06:18     from pkg_resources import load_entry_point
11:06:18   File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 2927, in <module>
11:06:18     @_call_aside
11:06:18   File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 2913, in _call_aside
11:06:18     f(*args, **kwargs)
11:06:18   File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 2940, in _initialize_master_working_set
11:06:18     working_set = WorkingSet._build_master()
11:06:18   File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 635, in _build_master
11:06:18     ws.require(__requires__)
11:06:18   File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 943, in require
11:06:18     needed = self.resolve(parse_requirements(requirements))
11:06:18   File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 829, in resolve
11:06:18     raise DistributionNotFound(req, requirers)
11:06:18 pkg_resources.DistributionNotFound: The 'nose==1.3.7' distribution was not found and is required by the application
11:06:18 CMake Error at /opt/ros/r2b2/share/ament_cmake_nose/cmake/ament_cmake_nose-extras.cmake:42 (message):
11:06:18   Failed to invoke nosetest: '/usr/bin/python3 /usr/bin/nosetests --version'
11:06:18   returned error code 1
11:06:18 Call Stack (most recent call first):
11:06:18   /opt/ros/r2b2/share/ament_cmake_nose/cmake/ament_add_nose_test.cmake:46 (_ament_cmake_nose_find_nosetests)
11:06:18   CMakeLists.txt:19 (ament_add_nose_test)

[wjwwood]

lgtm too, @nuclearsandwich feel free to merge when ever you're ready