Replace iallocator's mreloc w/ change-group and node-evac

This patch removes all occurrences of the “multi-relocate” iallocator
mode. Commit 25ee7fd8 updated the design document and introduced
separate modes, “change-group” and “node-evacuate”. The constants aren't
removed yet as they're still used by htools.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>