[Service Fabric] resource-manager/Microsoft.ServiceFabric/stable/2018-02-01/cluster.json doesn't follow standard schema as other rest api specs

The Cluster definition has a schema which doesn't match other azure rest api specs and should be as following:
`{
"Cluster": {
"type": "object",
"description": "The cluster resource\n",
"properties": {
"properties": {
"x-ms-client-flatten": true,
"$ref": "#/definitions/ClusterProperties",
"description": "The cluster resource properties"
}
},
"allOf": [
{
"$ref": "#/definitions/Resource"
}
]
},`

the Azure portal IAASEXP team uses these specs to generate models for the UI.

Open resource-manager/Microsoft.ServiceFabric/stable/2018-02-01/cluster.json and inspect the Cluster definition against the schema shown in the issue and other Azure REST API specifications. Done means the definition follows that structure, including the Resource reference and ClusterProperties reference, so the IAASEXP team can generate UI models from it.

Written by the indexing model from the issue text.