replace URL query with payload in HTTP request

[dmitsh]

This PR is a step one in adding support for block topology.
In this PR we replace the use of URL query in HTTP request for specifying provider and engine with the use of payload.
We also change the structure of the payload to contain provider credentials and engine parameters.
A sample payload will look like that:

{
  "provider": {
    "name": "aws",
	"creds": {
      "access_key_id": "id",
      "secret_access_key": "secret"
    }
  },
  "engine": {
    "name": "slurm",
	"params": {
	  "plugin": "topology/block",
	  "block_sizes": "30,120"
	}
  },
  "nodes": [
    {
      "region": "region1",
      "instances": {
        "instance1": "node1",
        "instance2": "node2",
        "instance3": "node3"
      }
    },
    {
      "region": "region2",
      "instances": {
        "instance4": "node4",
        "instance5": "node5",
        "instance6": "node6"
      }
    }
  ]
}