When job.spec.completions is nil, only
one task needs to succeed for the job to succeed,
and parallelism can be scaled freely during runtime.

Added tests.

Release Note:

This causes two minor changes to the API.

First, unset parallelism previously was defaulted to be
equal to completions.  Now it always defaults to 1 if unset.

Second, having parallelism=N and completions unset would previously
be defaulted to 1 completion and N parallelism.
(this is not something we expect people to do, though)
Now, no defaulting occurs in that case, and the job's
behavior is different (any completion causes success).