Review Request 35239: Update the JSON model for Resources to display their revocablility attribute.

[Jiang Yan Xu]

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https://reviews.apache.org/r/35239/
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Review request for mesos and Vinod Kone.


Bugs: MESOS-2776
    https://issues.apache.org/jira/browse/MESOS-2776


Repository: mesos


Description
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See summary.


Diffs
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  src/common/http.cpp 2ac7fba7a3aac913540f1b09768777393b79284a 
  src/master/http.cpp f8ac30934352db859e73819e0656a70047bb0dc5 
  src/tests/common/http_tests.cpp f087b2313a13c3199b70b3d7feb728e1449a52e7 

Diff: https://reviews.apache.org/r/35239/diff/


Testing
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Added a test.

Also tested with a master/slave pair.

e.g., The following state.json output shows a slave using a fixed estimator 
with `cpus:1;mem:512` revocable resources.
```json
  "slaves": [
    {
...
      "resources": {
        "cpus": 24,
        "disk": 454767,
        "mem": 71322,
        "ports": "[31000-32000]"
      },
      "total_resources": {
        "cpus": 24,
        "cpus{REV}": 1,
        "disk": 454767,
        "mem": 71322,
        "mem{REV}": 512,
        "ports": "[31000-32000]"
      },
      "used_resources": {
        "cpus": 0,
        "disk": 0,
        "mem": 0
      }
    }
  ],
```

Note that `resources` only looks at the resources from SlaveInfo while 
`total_resources` reads Master::Slave::totalResources.


Thanks,

Jiang Yan Xu