Separate OpNodeEvacuate.mode from iallocator

Until now the iallocator constants for node evacuation
(IALLOCATOR_NEVAC_*) were also used for the opcode. However, it turned
out this was due to a misunderstanding and is incorrect. This patch adds
new constants (with the same values) and changes the affected places.
Fortunately the RAPI client already used good names, so no changes are
necessary.

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

Signed-off-by: Michael Hanselmann <hansmi@google.com>