Implements PUT /api/resource_classes/ID to update the Flavors

This will delete all existing flavors and replace them with those in the
PUT body. Addresses https://github.com/tuskar/tuskar/issues/50

Testing please :)

If you run python sample_data.py - then you can test with:

    PUT http://0.0.0.0:6385/v1/resource_classes/1

    {
        "service_type": "foo",
        "name": "test-foo",
               "flavors": [
                    { "name" : "foo",
                      "capacities" : [
                         {   "name": "cpu", 
                             "value" : "1",
                              "unit" : "count" }, 
                         {   "name": "memory",
                             "value" : "1",
                             "unit" : "MiB" },
                         {   "name": "storage", 
                             "value" : "1",
                             "unit" : "GiB" }
                      ]
                    }
               ]
    }