Default Job completions to parallelism and add acceptance test

[stinkyfingers]

Description

Job completions value should default to the parallelism value if unset (per discussion w/ Vivek and K8s Job docs). Adds an integration test that asserts that Job completions and parallelism values are as expected.

Fixes # 1865 - this is the 3rd item listed in this issue.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Chore (documentation addition or typo, file relocation)

Testing

• New tests were added with this PR that prove my fix is effective or that my feature works (describe below this bullet)
• This change requires no testing (i.e. documentation update)

Documentation

• All added public packages, funcs, and types have been documented with doc comments
• I have commented my code, particularly in hard-to-understand areas

Final Checklist:

• I followed standard GitHub flow guidelines
• I have performed a self-review of my own code
• My changes generate no new warnings