Fix generalized relocate mode of IAllocator

The patch which generalized the IAllocator was half-true: it actually
put the selection of the node inside the IAllocator, so callers were not
able to specify replace primary node.

This patch does:
  - split the arguments to the constructor in three sets: mode and name
    are always passed, and then we differentiate between allocation
    parameters and relocation ones
  - add a new relocate_from option to the IAllocator constructor which
    is a list of nodes we want to move the instance off
  - rename the 'nodes' argument in the request object to 'relocate_from'
    since this is clearer and is not confused with the result field also
    called 'nodes'

Reviewed-by: ultrotter