Issue 49640 - Cleanup plugin bootstrap logging
Bug Description: We add PBKDF2_SHA256 password storage schema two times. During:
1. the dse.ldif parsing;
2. the bootstrap plugin operation.
It causes the error to appear during the startup.
Fix Description: Make plugin_setup() function report the error to TRACE log level
if the plugin already exists. We will report the error in ERR log level during
the config bootstrap anyway (code path for the 1st option from bug description).
For 2nd option, report the error to TRACE if it is 'already exist' issue
and to ERR if it is any other case.
Make the plugin_setup returns more consistent.
Reviewed by: mreynolds, mhonek (Thanks!)