fix typo in clientver arg name
refactor checkout logic, pushing the multi-repo logic up to the driver
remove default version handling, all clients are now specifying their version
fix another clientver typo
merge output code now that it's unconditional
p_get_runnable_master updates

1) remove debug logging statement
2) add preliminary support for server specified platform, repo, and branches
refactor driver code to pull the build step specification up a layer
clientver 2: change get_runnable_pull so that the order of the first few fields match get_runnable_master
clientver 2: merge client driver-pull.sh into driver.sh