Add ability to upload test results

Split refstack-client into two subcommands: upload and test.
The 'test' command will run Tempest like before, while 'upload'
takes in a result file argument and will upload it to the Refstack
API url. Testing with refstack-client now saves a JSON result file
containing the content of what will or would be posted to the
Refstack API.

Change-Id: I28bcb75b51b77872f39e144ae8ffa7e64b26b233