Fixed possible pep8 failure due to pyflakes bug

There is a bug in pyflakes that may trigger F811 violation when import
is not used in a file before its redefinition in local class method
scope, even though it's used in other classes later in the same file.

Change-Id: Ie611e2ff93c59b01a2695c3d944cfe118f874547
Related-Bug: #1297161
Use messaging_conf fixture configuration by default

This simplifies usage of messaging_conf fixture, reduces explicit
cleanup and option registration code in unit tests.

Change-Id: If1dea548c4a58abfc8ce4967710a061c9058628c
Fixed inconsistent EventletContextManagerSpawnTest failures

The test spawns a greenthread and expects it to complete after
eventlet.sleep(0) call. The problem is that if there are other
greenthreads currently running, sleep(0) may switch to unexpected
thread, which may result in test thread not being invoked at all.
Instead of assuming no other green threads are running at the moment
of test execution, explicitly wait() for the test thread.

Change-Id: Id45307860733658638b74ed3a713493df8c1080d
Closes-Bug: #1282706
Merge "Fixed inconsistent EventletContextManagerSpawnTest failures"
Merge "Fixed possible pep8 failure due to pyflakes bug"
Merge "Use messaging_conf fixture configuration by default"