htools: rework the algorithm for ChangeAll mode

I think I've identified the problem with the current ChangeAll
mode. The current algorithm works as follows:

- identify a new primary by choosing the node which gives best score
  as new secondary
- failover to it
- identify a new secondary by choosing the node which gives best score
  as new secondary

This means that the future primary is 'fixed' after the first
iteration, leaving to possibly suboptimal results. This patch changes
the algorithm to do what, in hindsight, seems the obvious thing to do:
- generate all pairs (primary, secondary)
- identify the pair that after the above sequence (r:np, f, r:ns)
  gives the best group score

This fixes some of the corner cases I've seen in relocation, but not
all; the remaining cases are related to multi-instance relocation and
while they can't be fixed in the current framework, the needed
rebalancing is much smaller than with the current algorithm.

The patch also fixes an issue with the docstring of another function.

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