labs: add options for config cmds

This patch adds two options to configuration commands (config/scripts.*).

-n <node_name>: change target node of command
-g <gui_type>: override GUI type (gui, headless, sdl) for this boot

The changed syntax is as follows:

cmd boot [-g <gui_type>] [-n <node_name>]
cmd snapshot [-n <node_name>] <snapshot_name>
cmd wait_for_shutdown [-n <node_name>]
cmd snapshot_cycle [-g <gui_type>] [-n <node_name>]
cmd init_node [-n <node_name>]

Change-Id: Id2673c6d504aed33f0259d5751084442e851684b
Merge "labs: add options for config cmds"