Namespace override with ROS2_NAMESPACE environment variable

[alpylmz]

This PR addresses the issue ros2/ros2#1259. The environment variable is chosen as ROS2_NAMESPACE. A design issue that needs to be discussed is if the ROS2_NAMESPACE variable will override all namespaces defined or will be added to the namespace. For example,

ROS2_NAMESPACE=/bar, node's namespace is /foo, should the overall namespace be /bar, or /bar/foo.

[fujitatomoya]

Do we need this change?

[alpylmz]

I just had some time to work on this, and I realized that the remap code is executed in every service and topic creation at the runtime. We could add the namespacing after remap's, but that means adding the same code to 5 files, client.c, node_resolve_name.c, publisher.c, service.c, subscription.c.
I think a much cleaner way would be leaving this pull request almost as is, changing the launch code to not to use remaps for namespacing but use a function that we may implement at this repo(to add additional namespaces in any kind to node), and merge both of the pull requests. What about this?

[alpylmz]

This PR stayed for a long time than it really should be. Is there anyone who can comment on?