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.
Add 'config' parameter to Git source build step to enable
git-config settings when running the low-level git commands.
RemoteShellCommand: add parameters to override user the command is run as
tryclient: support commas in property CLI

This adds a '--property' flag which contains single a property setting
and can be set multiple times (allowing for comments in property values).
Merge branch 'try_properties' of git://github.com/andrewjcg/buildbot
Merge branch 'git_config' of git://github.com/andrewjcg/buildbot
Pass tryclient properties to getAvailableBuilderNames call
Merge branch 'try_client_available_builder_names' of git://github.com/andrewjcg/buildbot

(with pyflakes fix)
don't print error message until after checking exception type
Bugfix for ticket #2370, ShellCommand now checks validity of args during config rather than build