Don't stream non-binary requests

Setting stream=True with requests can lead to issues
with not closing the connection so the urllib3 connection
pool is not freed up, so only set stream=True if making
a request with application/octet-stream content-type.

See the body-content-workflow and keep-alive sections
in the requests docs here for more information:

http://docs.python-requests.org/en/latest/user/advanced/

Note that commit dbb242b changed the response body_iter
code to potentially return a six.StringIO object rather
than the old ResponseBodyIterator class and since the
images client code is not converting the body_iter into
a dict using json.loads, we have to do that directly
in the _request method where the body_iter is
constructed.

Co-authored-by: Flavio Percoco <fpercoco@redhat.com>

Change-Id: I86572b69c4511f933c9676108190271874346302
Partial-Bug: #1341777