Allocator framework, 1st part: allocator input generation

In preparation for the introduction of automatic instance allocator,
this patch adds an allocator simulation opcode, that based on the input
parameters, will return either the input message to the allocator
(implemented) or the result of the allocator run (not yet implemented).

This allows algorithm tests against simulated allocations and the
current cluster state.

The patch adds the following:
  - a function that generates the generic cluster information for the
    allocator
  - a function that generates the 'new instance' information
  - a function that generates the 'replace_secondary' information

These three functions will be used by the allocator framework later to
generate the actual information for the external algorithms. Currently
we just return the json-serialized text.

Reviewed-by: imsnah