Cleanup replace-disks modes and options

In 1.2, due to the md+drbd7 legacy, we had a complex choice of replace
modes, and the new drbd8 modes where forced into this syntax, with some
complicated rules of transition from one mode to another (if REPLACE_ALL
but not new node passed, switch to REPLACE_SEC, etc.).

This patch cleans this situation by making a clear separation between
the two main modes:
  - replace on current nodes (with the two sub-cases on primary and on
    secondary)
  - change to a new node (either via manually specified node or via
    iallocator)

Reviewed-by: imsnah