NiFi and NiFi-Registry 1.15.0 support
- Moved testing to secured single user by default for NiFi
- Consolidated clients to the 1.15.0 release version for both NiFi and Registry
- Updated logic to support changed working modes in tests
- Corrected minor functional issues for 1.15 in login handling, SSL certs, Parameter Updates and Testing
- Backwards compatibility and Regression tested as requiring no breaking changes