Fixes and tests for remappings passed to Node actions

[dhood]

Some issues I noticed while working on #135 (kept as separate commits).

Most significant change is that, without this PR, something like:

def generate_launch_description():
    os.environ['TOPIC_NAME'] ='chatter'
    return LaunchDescription([
        launch_ros.actions.Node(
            package='demo_nodes_cpp', node_executable='talker', output='screen',
            remappings=[
                (EnvironmentVariable(name='TOPIC_NAME'), [
                    'new_', EnvironmentVariable(name='TOPIC_NAME')])
            ],
            log_cmd=True,
        ),
    ])

would not remap properly, and would output:

[INFO] [launch]: process[talker-1] details: cmd=[/home/dhood/ros2_ws/install/demo_nodes_cpp/lib/demo_nodes_cpp/talker, <launch.substitutions.environment_variable.EnvironmentVariable object at 0x7f239d09ea20>:=['new_', <launch.substitutions.environment_variable.EnvironmentVariable object at 0x7f239d09e9b0>]], cwd='None', custom_env?=False

whereas now it evaluates the substitutions and gives:

[INFO] [launch]: process[talker-1] details: cmd=[/home/dhood/ros2_ws/install/demo_nodes_cpp/lib/demo_nodes_cpp/talker, chatter:=new_chatter], cwd='None', custom_env?=False

This PR currently targets the branch used for #135 because it builds on top of it; will be retargeted at master once that PR's merged.

[dhood]

this can be reviewed but there might be duplicate comments from #135 since they have similar approaches; if review comments are added on that PR I will propagate the changes to this one.

[wjwwood]

lgtm, thanks for fixing that up!

[dhood]

Thanks for the quick review @wjwwood. I added two simple commits since then.

Standard CI (for launch packages):

• Linux
• Linux-aarch64
• macOS
• Windows

Repeated tests looking for flakiness:

• Linux
• Linux-aarch64
• macOS
• Windows