Consider DC-level config when validating numToken updates in webhook (fixes #1222)

[olim7t]

What this PR does: Instead of only comparing the top-level config (which can fail if fields are missing), compute and compare the numToken of each individual DC, using the inheritance and defaulting rules. This also covers cases that were not handled before, like config being nil on one side but not the other.

Which issue(s) this PR fixes: Fixes #1222

Checklist

• [ ] Changes manually tested
• [x] Automated Tests added/updated
• [ ] Documentation added/updated
• [x] CHANGELOG.md updated (not required for documentation PRs)
• [x] CLA Signed: DataStax CLA

[adejanovski]

@olim7t @burmanm, what's the status on this PR?