Add pod liveness check before starting cassandra server.

[garrynigel]

What this PR does:

Which issue(s) this PR fixes: Fixes #759

Checklist

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