Multiplex:  reading node parents and variables in determinisc way + simple (with paths) output for multiplex plugin

The variables (the order of output) is important to present, when I'm debugging the multiplex, I want that the same YAML file give me the same output by running the command `avocado multiplex foo.yaml --show-path --contents`. So I can diff the outputs.

**Note that I'm still investigate the bug in variants.**

here's a diff  I have by running two or more executions of `/scripts/avocado multiplex -c example.yaml`:


<pre>
-Variant 72:    /root/hw/cpu/arm, /root/env/prod, /root/os/linux/mint, /root/os/win/win8, /root/hw/disk/virtio         │                                                                                                                      
+Variant 72:    /root/hw/cpu/arm, /root/os/linux/mint, /root/env/prod, /root/os/win/win8, /root/hw/disk/virtio         │                                                                                                                      
     bsod: false                                                                                                       │                                                                                                                      
     cpu_CFLAGS: -mabi=apcs-gnu -march=armv8-a -mtune=arm8                                                             │                                                                                                                      
     debug_CFLAGS:  
</pre>

List of changes:

* Using OrderDict to keep things deterministic.
* Change the output of variants in multiplex plugin, just show the node path, don't show the internal structure.