Dual OCP and nonOCP compatibility adjustments

[greyerof]

Fixes to make it compatible with SNO and OCP, but also to prepare samples for
more securityContext's fields checks.

Specific to special-dp pods:

• Added template for replica number in special-dp to make it compatible with
  SNO clusters.
• Affinity rule updated to use "operator: Exists", since those labels don't
  have any value in real OCP clusters.

Other changes:

• Added pod/container securityContext fields depending on whether the test
  pods are deployed in Kind or Openshift. For OCP, most fields are not needed
  as they're set by SCC policies, whereas in Kind they must be explicitly set.
  Otherwise, some access-control test cases might fail.
• Moved REPLICAS env var setting to init-env.sh script.
• Avoid creating file in /tmp to prepare pods to pass the
  readOnlyRootFilesystem check.

[greyerof]

Everything works except for the "check results" step, where the failing check is now passing (which is the purpose of some of the modifications):

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| TEST_CASE                                                  EXPECTED_RESULT     ACTUAL_RESULT |
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| access-control-security-context                                     FAILED            PASSED |
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