Fix build number guessing

[hughsaunders]

Currently the build number is guessed by querying for the next build number immediately before initiating a build. This is super racy, and may be defated by other users starting builds, or by builds being started in parallel.

There should be a mechanism for initiating a build and extracting the build number of the build that was started. It may be necessary to hit the jenkins api directly rather than using the python wrapper.