Make gnt-node list -o(p|s)inst_list output stable

Make gnt-node list -o(p|s)inst_list output stable

Currently, both the Python and Haskell code return the internal
instance list unsorted, which means the output can vary depending on
the phase of the moon (well, the Haskell code actually uses internally
a tree, sorted by the instance name, but it's implementation detail).

By adding an explicit sort step, we guarantee both stability of the
output and consistency between the two query paths.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>