Add new opcode to evacuate node

This new opcode will replace LUNodeEvacStrategy, which used to return a
list of instances and new secondary nodes. With the new opcode the
iallocator (if available) is tasked to generate the necessary operations
in the form of opcodes. This moves some logic from the client to the
master daemon.

At the same time support is added to evacuate primary instances, which
are also evacuated by default.

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