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.