Fix iotlab

[rienafairefr]

The iotlab common command had some problems, probably something changed when simplifying something,

• for plot subcommand: oml_plot_tools import hierarchy needed to go one level deeper (use import oml_plot_tools.consum instead of import oml_plot_tools then use oml_plot_tools.consum.main)
• wrong prog: iotlab node --help or iotlab node showed wrong usage prog(sys.argv[0]) (iotlab only instead of iotlab node
• iotlab subcommand was not tested for the "tools" subcommands (serial/sniffer/plot/ssh)

PS: it might be a good roadmap to migrate in the future to something that better handle hierarchy of subcommands, like click or others, instead of argparse which is a bit limited in that regard

[rienafairefr]

oml_plot_tools needed to be changed a bit (see latest commits to master there) to be compliant with the new iotlab command, but iotlab plot and all the others works fully now. There's still some docs to write up a bit before 2.6.0 release