Add latent buildslave for OpenStack

The buildslave has limited functionality for interacting with
OpenStack's Nova component. Booting and terminating instances
works correctly, but no interfacing with the network part.
Move buildslave.py into buildbot.buildslave
Move openstackbuildslave.py into buildbot.buildslave.openstack
Add blurb in release notes
Add documentation on how to use new Openstack latent slave
Use callable instead
Use camelCase instead of underscores
Drop creating and using ConcreteBuildSlave

Not using an abstract slave, so do not need a concrete version. Some of
the lines are a bit long; this will be fixed later.
Use keyword args in test

Due to the number of required arguments, use keywords for them to be
clearer about what's being specified. Also changed the flavor to be an
integer because the docs say to pass the flavor ID, not name.
Update docs

Use keyword arguments in the examples and list the keyword arguments
with a brief description on each.
Include the image UUID in the log

The instance name was included in the log message, but this is identical
to the slave name. Instead, include the image UUID as this could
actually help in debugging.
Merge branch 'openstack' of git://github.com/crayinc/buildbot
fix pyflakes