Make the build order file configurable via a CLI flag

[tacaswell]

Currently there is a single build order which is a mix of standard Scientific Python, some web related Python, and source from the bluesky suite.

Most people will not be interested in installing the bluesky suite (although if you are, we can talk about that too!). It would be good to first make the file used configurable (and maybe take a list to be run in order) so that multiple stacks can be supported.