Generalize the replace_secondary mode in iallocator

Currently the replace_secondary mode is too restrictive. This patch
changes this to a general 'relocate' mode where the node(s) to be
changed are specified via a new key in the request dict ('nodes') so
that we can change any of the instance's nodes.

Note that for the relocate mode, len(nodes) == required_nodes, so the
required nodes field is redundant, but it is provided for consistency
with the allocate mode.

Reviewed-by: ultrotter