Update demo nodes to enable loaned messages by default

[orecham]

Brief Description

The examples need to be run with this environment variable set (ROS_DISABLE_LOANED_MESSAGES=0) to enable the loaning API.

If there is a way to set this configuration in code, this should be done to make running the examples simpler and also provide a reference for users to see how to do this themselves.

[elBoberido]

@orecham I'm not sure but I think environment hooks can be used for this