commands: report firewall now supports SNAT

with this minor modification it is possible to use SNAT instead
of MASUQERADING on the frontend. The command to set the rule is:

rocks add firewall host=localhost action=SNAT chain=POSTROUTING \
    service=nat rulename="10 snat" flags="--to-source 123.1.2.3" \
    output-network=public protocol=all

Reported-by: Juan Eduardo Ramirez <juaneduardo.ramirez@upr.edu>
node.xml: fix all the nodes with the new syntax to support reconfiguring
commands: os can be linux and sunos (not solaris)

I guess this is a leftover from some renaming of the solaris os
Solaris does not even fit in the table column for OS
commands: save the old value of the attribute when you change value

When the user change the value of an attribute called 'name' the
old value is saved in an attribute called 'name'_old
This is used by the reconfigure procedure.

This code is pretty bad, lots of code duplication. I didn't have
time to fix previous structure of command to avoid code duplication.