Add multiple min/max specs in instance policy

Now instance policies can contain more than one min/max specs.  This is the
main element of the "Constrained instance sizes" section in the
"Partitioned Ganeti" design doc.

This is a big patch, but changing the type of a configuration item requires
to change all the code that handles it.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>