Documentation updates to provide alternatives to usage of TLS Toolkit

mailing list discussion:

• https://lists.apache.org/thread/vn1nzobtz4fh7fs461sgg8jj9zygrk0f

Eventual goal is the deprecation of TLS Toolkit [1] for targeted NiFi 2.0 release, in order to address maintenance concerns.

Generation of equivalent keystores and truststores for single development node is covered by automatic self-signed certificate generation added in NiFi 1.14.0 [2]. Kubernetes cert-manager / organization-specific Certificate Authorities / Let's Encrypt service provide coverage for keystore needs in production deployments.

For development clusters, OpenSSL and Java Keytool provide another means to generate NiFi-ready keystores to be used for secured communication between cluster nodes. Documentation of needed "openssl/keytool" command sequences should fill needs currently provided by TLS Toolkit.

Probable scope would include touches of Admin Guide (administration-guide.adoc), Toolkit Guide (toolkit-guide.adoc), and Walkthroughs (walkthroughs.adoc). Follow-on work would target the removal of the current TLS Toolkit.

[1] https://github.com/apache/nifi/tree/main/nifi-toolkit/nifi-toolkit-tls
[2] https://issues.apache.org/jira/browse/NIFI-8403