Make swift post output an error message when failing

swiftclient.shell.st_post was ignoring the result dict
returned from SwiftService.post and therefore giving no
indication when posts failed e.g. due to invalid auth
credentials.

This patch ensures that SwiftService always returns a
result_dict from post() and then checks the result dict.
On failure the shell now outputs the ClientException
message and exits with error code 1.

Also adds unit tests and cleans up some unnecessary mocked
return values in existing tests.

Closes-Bug: 1383243

Change-Id: I111150eb3d026c8d01c2cac13d3613ca7304e5b9
Merge "Make swift post output an error message when failing"