Add BOSH user credentials support to Genesis

[wayneeseguin]

Implement support for personal BOSH user credentials via environment
variables (BOSH_USER/BOSH_PASSWORD), allowing operators to use their
own credentials instead of shared admin credentials from Vault.

Key changes:

• Add user credential detection and validation in Service::BOSH::Director
• Prioritize user credentials over admin credentials in credential detection
• Support dual credential format (user/client) for BOSH CLI compatibility
• Update Genesis::Env to check for user credentials first
• Add comprehensive documentation and examples

Benefits:

• Improved security with individual user authentication
• Better audit trails showing who performed each action
• Reduced credential sharing across teams
• Maintains full backward compatibility

The implementation follows BOSH CLI standards and all existing workflows
continue to work unchanged. When BOSH_USER and BOSH_PASSWORD are set,
Genesis automatically uses them and sets both user and client credential
formats for maximum compatibility.

Note: UAA admin credential storage remains the responsibility of the
BOSH kit, not Genesis core.