RAPI: switch evacuate node to the new model

This patch removes the last use of the old-style OpEvacuateNode. It also
fixes the dry-run mode for this RAPI resource - the dry-run parameter
was not used at all before.

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