Add tab-completion to move_joint script

[adolfo-rt]

Figuring out the name of a joint is cumbersome, and for users not familiar with a robot it can be hard to remember strings like arm_left_4_joint. Further, move_joint can't move every robot joint, but only those with an active joint trajectory controller.

Since play_motion knows about the available controllers (hence the joints they control) it should be possible to expose a ROS service for querying the joints it can currently act upon.

Tab completion could be built on top of this service.

[po1]

Tab completion on top of services is slow, and can completely block the terminal if the node is not responding. Though I understand the need for this feature, I am not too fond about building it on top of a service...

[adolfo-rt]

An alternative would be to have the controller updater write the available joints to a ros parameter or file. List updates should be infrequent and fast .We'd no longer risk blocking indefinitely, as happens with services. Other lightweight alternatives to consider?.

[po1]

Writing to a ros parameter seems the least bad of the list, but I still don't like it too much.
From a very primitive benchmark, I have the following results:

call	time [s]
rosservice call	0.9
rosparam get	0.09

Since there is no broad usage of shared files in ROS, I don't feel too good about writing that to a file either. It seems less portable and more prone to problems (file permissions, R/O system...)

[adolfo-rt]

I'll see when I can schedule a time slot for this one then, and draft a rosparam-based implementation.