[rackspace] fix JSON error parsing

Prevent an error in extracting the error message from the JSON response
from discarding the successfully parsed JSON response body.

For example, the parsed error response body from a failed PUT request to
store a SLO manifest object would be:
  { 'Errors' => [['path', 'error'], ['path', 'error']] }
[rackspace|storage] add methods for SLO support

Adds #put_static_obj_manifest and #delete_static_large_object methods.
[rackspace|storage] add #put_dynamic_obj_manifest

Renames the current #put_object_manifest method to better differentiate
this from the new #put_static_obj_manifest method. #put_object_manifest
has been retained for backward compatibility.
[rackspace|storage] patch #delete_static_large_object for Swift v1.8

This commit addresses an issue where the Content-Type header is not
being set correctly in the response. This has been fixed in Swift v1.9.
openstack/swift@ad24cde120efe73fe5a9b866f74c7d3c766aa4cb

The tests have also been updated so they don't rely on changes in Swift v1.9
that affect the response body when errors are reported.
openstack/swift@6768d5b4be3e3922d8077b6c603586c1b7df2d07