Add ReadOnlyRootFilesystem to Security Context

[david-yu]

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Changes proposed in this PR:

• Re-raise PR 2789 which introduces readonlyfs security context
• Will be merged after Consul K8s 1.2.2 is released

How I've tested this PR:

How I expect reviewers to test this PR:

Checklist:

• [ ] Tests added
• [ ] CHANGELOG entry added

[hashicorp-cla]

All committers have signed the CLA.