Rework node role changes

There have been many bugs in gnt-node modify. Let's try to introduce
some more.

This patch reworks the node role changes from tracking the flag changes
to completely overwriting the flags based on the new role. This paves
the way for (in 2.4 or later) moving to a single attribute for nodes.

We compute the old role and the new role based on the required changes
and whether we need to auto-promote. Once this is done, the body of the
Exec() function becomes trivial (there's more code related to output
formatting than the node flag changes).

Another advantage of the new version is that the entire flags are
overwritten, and that all are changed at the same time, making it
impossible (harder?) to have partial updates.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>