driver: combined JTA should perhaps be namespaced (multi-group)

[gavanderhoorn]

Triggered by #185: besides the joint_trajectory_action servers that live in the namespaces for each of the configured motion groups (in a multi-group setup), there is an instance that is not namespaced. This JTA accepts goals for all joints configured.

As all other JTAs are namespaced according to the ros parameters for the nodes, it might make sense to namespace the all-joints JTA as well, but without any reference to the groups in the topic name.