BE: RBAC: Subject type/value is unintended to be optional

[wernerdv]

• Breaking change? (if so, please describe the impact and migration path for existing application instances)

What changes did you make? (Give an overview)

• Resolves RBAC: Support Active Directory #54
• Resolves BE: RBAC: Subject type/value is unintended to be optional #716
• minor refactoring

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How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• No need to
• Manually (please, describe, if necessary)
• Unit checks
• Integration checks
• Covered by existing automation

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

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[wernerdv]

@Haarolean I checked locally and with these changes RBAC with AD works as expected for both types (user, group).

Parameter value rbac.roles.name can be anything.

If need I can add integration tests with Active Directory.