Fix request_id type on Python 3: use text (Unicode)

generate_request_id() was previously modified by the change
I389cf20e4c999c9bb576a42388497852f8ba27ec to return a bytes string
instead of a text string. According to the author of the change, it
was a mistake (the right fix is to update the unit test, not modify
the tested function).

Having a request_id as a byte string is not convenient on Python 3.
For example, it's a bug to write '"id:%s" % request_id', a
BytesWarning exception is raised when python3 is run with -bb command
line option. It not possible to write str+bytes neither.

Right now, oslo_log emits logs with b'...' which is unexpected. I
prefer to fix the issue in oslo_context rather than working around
the issue in oslo_log. Example of log emitted on Python 3:

"WARNING glance.api.v2.tasks [b'req-83a6...' 54492... - - -] ..."

This change depends on changes in Glance, Neutron and oslo.middleware
which prepares unit tests to support bytes and str request_id. So
their unit tests will work on the old and new versions of
oslo.context. Changes on unit tests are required for the master
branch but also Liberty because their python34 check job is voting on
Liberty and master.

This change depends on the following changes:

* change Ia8578ecab1b1f1bf21020c91290458efad646cd9
* change Ia5203581db120bb4f8e24fd752d6ae14e6558505
* change I872c268ef5f17dc2c1fe0ce8aa7e8af4ebc1d757
* change Ia4df8f05d5746887eb99bbf368d29e8d30a3a90a

Closes-Bug: #1526662
Change-Id: If48ee7f4d1c113f1f26b3b1698c6b055807b950f

oslo_context/context.py

@@ -29,7 +29,7 @@ _request_store = threading.local()
 
 
 def generate_request_id():
-    return b'req-' + str(uuid.uuid4()).encode('ascii')
+    return 'req-%s' % uuid.uuid4()
 
 
 class RequestContext(object):