htools: add node-evacuation of DRBD all nodes

This is much more complicated than either change primary (which is
trivial) and change secondary (which is also simple).

The main strategy is:

- choose a new secondary (that will become the final primary) and
  replace-disks to it
- failover
- choose a new secondary again (this will be the final secondary)

However, if the original primary was offline, then we first have to
failover to the secondary of the instance.

Note: currently this evacuation mode, while producing correct
evacuation results, does not result in optimal placement of the
instance; it will need attention and possibly an algorithm change.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>