Can't get output of SoftwareDeployment resource when using HEAT_SIGNAL

When using HEAT_SIGNAL option, the output of SoftwareDeployment resource can't be returned correctly. All output attributes such as 'deploy_stdout, deploy_stderr, deploy_status_code' will be null.

In 55-heat-confg.py, the output of hooks(signal_data) will be dumped to JSON object before passed to heat client. However, heat client will dump it again before invoking _http_request.

For example, if the original output is:
{'deploy_status_code': 0, 'stderr': 'test-stderr', 'stdout': 'test-stdout'}

The first JSON dump result will be:
{"deploy_status_code": 0, "stderr": "test-stderr", "stdout": "test-stdout"}

The second JSON dump result will be:
"{\"deploy_status_code\": 0, \"stderr\": \"test-stderr\", \"stdout\": \"test-stdout\"}"

This is not a valid input for requests.request() for 'data' parameter. Therefore, the 'body' will be empty when heat-api accept the request and all output of SoftwareDeployment resource will be lost.