Rework evacDrbdSecondaryInner to take the imove

Instead of hard-coding the instance move operation, let's accept is as
a parameter from the caller. This is the single thing that makes this
DRBD-specific, so removing it will help for future
patches. Furthermore, this abstract the imove op type into a single
place instead of two (the inner function and the nodeEvacInstance).

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