Restore min/max for cores and memory

[Zarquan]

Separate the requested and offered values for the generic-compute cores and memory and restore the min/max values for the requested set.

resources:
  compute:
  - type: uri:generic-compute
    cores:
      requested:
        min: 4
        max: 8
    memory:
      requested:
        min: 16
        max: 24
        units: GiB

New rational - if/when the service offers more resources than requested, the user can specify a max value on some nodes to shift the extra allocation to other modes.