add messages as cmake output that displays the discovered rmw impls and the default

[wjwwood]

Looks like this:

...
-- Found OpenSplice: ...
--
-- Found these rmw implementations:
--   'rmw_connext_cpp'
--   'rmw_connext_dynamic_cpp'
--   'rmw_opensplice_cpp'
--
-- Using default rmw implementation: 'rmw_opensplice_cpp'
--
-- Added test 'lint_cmake' to check CMake code style
...

[wjwwood]

CI: http://ci.ros2.org/job/ci_linux/1317/

[wjwwood]

Without FastRTPS because it's failing: http://ci.ros2.org/job/ci_linux/1318/

[wjwwood]

CI is passing, so I'm going to merge this. I'll address any comments about style if they come in afterwards.