Create a parameter called tox_upper_constraints. The parameter points
to a file on the system. If the file exists, it will be passed to the
tox invocation in the UPPER_CONSTRAINTS_FILE environment variable.
This allows a global setting in a base job of a known file to be the
UPPER_CONSTRAINTS_FILE file, or for that to be overridden on a per-job
basis. For people who do not know or care what an
UPPER_CONSTRAINTS_FILE file is, the behavior should be a no-op.
Also, collapse tox_environment and tox_environment_defaults.
zuul already does variable merging on parent and child jobs, so we don't
need to do it in the playbook.
While we're in there, clean up a few small things in the yaml. Notably,
jlk says we should not put variables in task names, and we don't need
quotes around strings if they don't have special characters.
Change-Id: Ibede59ca6f85914952da175c77ea29391ef9a88c
022832a1d7