Errors when using build_pkg/test_pkg

[dhood]

Whenever I try to use the build_pkg or test_pkg verbs on ament_cmake packages instead of build/test, I get the following error:

$ ament build_pkg .

Process package 'pendulum_msgs' with context:
--------------------------------------------------------------------------------
 source_space => /Users/deanna/ros2_ws/src-master/ros2/demos/pendulum_msgs
  build_space => /Users/deanna/ros2_ws/src-master/ros2/demos/pendulum_msgs/build/pendulum_msgs
install_space => /Users/deanna/ros2_ws/src-master/ros2/demos/pendulum_msgs/install
   make_flags => None
  build_tests => False
--------------------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/deanna/ros2_ws/src-master/ament/ament_tools/install/bin/ament", line 9, in <module>
    load_entry_point('ament-tools==0.0.0', 'console_scripts', 'ament')()
  File "/Users/deanna/ros2_ws/src-master/ament/ament_tools/install/lib/python3.5/site-packages/ament_tools/commands/ament.py", line 88, in main
    rc = args.main(args)
  File "/Users/deanna/ros2_ws/src-master/ament/ament_tools/install/lib/python3.5/site-packages/ament_tools/verbs/build_pkg/cli.py", line 270, in main
    context = get_context(opts)
  File "/Users/deanna/ros2_ws/src-master/ament/ament_tools/install/lib/python3.5/site-packages/ament_tools/verbs/build_pkg/cli.py", line 276, in get_context
    return create_context(opts)
  File "/Users/deanna/ros2_ws/src-master/ament/ament_tools/install/lib/python3.5/site-packages/ament_tools/verbs/build_pkg/cli.py", line 383, in create_context
    ce = build_type_impl.extend_context(opts)
  File "/Users/deanna/ros2_ws/src-master/ament/ament_tools/install/lib/python3.5/site-packages/ament_tools/build_types/ament_cmake.py", line 59, in extend_context
    ce = super(AmentCmakeBuildType, self).extend_context(options)
  File "/Users/deanna/ros2_ws/src-master/ament/ament_tools/install/lib/python3.5/site-packages/ament_tools/build_types/cmake.py", line 88, in extend_context
    force_cmake_configure = options.force_cmake_configure
AttributeError: 'Namespace' object has no attribute 'force_cmake_configure'

This happens for me regardless of the options passed, but I think the fix might depend on the following:
Is it expected that cmake options can be passed to ament_cmake packages when using build_pkg, as is possible with build?

If the answer's no, then I think it might be sufficient to provide default values for the cmake args to avoid the error here

If the answer's yes, then I think cmake args, in addition to the ament_cmake args, have to be added to the build_pkg subparser when an ament_cmake package is detected. (I made an attempt at this with #81)

[dirk-thomas]

Addressed by #152.