Preflight check for password_secret

[todvora]

Our users often encounter hard to understand problems and errors when they misconfigure password_secret and don't use the same value for all (data/server) nodes.

We should be able to recognize this issue early, during preflight checks.

First idea:

• Check cluster config for a specific encoded entry
• If the entry exists, try to decode it. If not found, create one
• If the decoding succeeds, verify that it contains expected and known value. Otherwise fail and stop boot with good error message.

Related issues:

• https://community.graylog.org/t/unable-to-start-graylog-after-update-from-6-0-9-to-6-1-x-illegalargumentexception-illegal-base64-character-3f
• https://community.graylog.org/t/ca-and-csr-issues/34607