Previously we could only set OSMO_TTCN3_BRANCH as environment variable
to build a test other than master.
This patch adds environment variables for all osmo-*-master images which
allow docker tests to be executed for an arbitrary commit.
The origin/ prefix from the git checkout command is removed so the
*_BRANCH variable doesn't have to contain branch names, but van also
contain arbitrary commits. This shouldn't have any adverse effect as we
only have one remote in the checkout.