Support branch input to unit test docker script

With the upcoming OpenBMC release, it will be the first time
an official branch is done to all of the repos. The CI
infrastructure needs to support building against this
new branch (as well as master).

Testing:
- Verified with no branch input it behaves as it always did
- Verified with a non-existent branch it behaves as it always did
- Verified with a valid branch in a single repo that it used it
  and defaulted to master for the rest

Change-Id: Ic71153483eb407f596afea1b748387b23bbc81fd
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
2 files changed