fix: LDAP init. issue when LDAP server is down

[donatello]

Description

At server startup, LDAP configuration is validated against the LDAP server. If the LDAP server is down at that point, we need to cleanly disable LDAP configuration. Previously, LDAP would remain configured but error out in strange ways because initialization did not complete without errors.

Motivation and Context

This can be seen easily when we start minio + ldap server together in docker-compose for e.g. using https://github.com/minio/minio-iam-testing/blob/main/docker-compose.yml

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (If yes, please add commit-id or PR # here)
• Unit tests added/updated
• Internal documentation updated
• Create a documentation update request here