multiplex: Be deterministic when reading nodes and variables.

When reading nodes and variables, it's important
to keep the order of the structure defined in original
multiplex file, to facilitate debug (with the multiplex plugin)
and to maintain the order of going into node parents.

So we're removing dicts and sets and using OrderedDict.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
avocado.plugins.multiplexer: Display path of nodes by default.

By default, it will now display the paths of the nodes, so
we're not going to show all that structure as it was before.

If you want to see the variables, just pass the option `--contents`
to the plugin.

Here's an example output:

Variants generated:
Variant 1:    /root/hw/cpu/intel, /root/hw/disk/scsi, /root/os/linux/fedora, /root/os/win/winxp, /root/env/debug
Variant 2:    /root/hw/cpu/intel, /root/hw/disk/scsi, /root/os/linux/fedora, /root/os/win/winxp, /root/env/prod
Variant 3:    /root/hw/cpu/intel, /root/hw/disk/scsi, /root/os/linux/fedora, /root/os/win/win7, /root/env/debug

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #189 from ruda/multiplex_ordered_dict

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