iotlab-support: use iotlab command to check credentials

Since the `iotlab-term` target uses `tmux` error messages are not really
printed, so it took me a while to find out why at some sites this target
wasn't working for me anymore.

If the IoT-LAB password was changed, just checking if `.iotlabrc`
exists isn't enough, so I use `iotlab-experiment` to check if I'm logged
in properly to prompt the password input in case I'm not.