Add configurable HTTP Server header for deception

Add SERVER_HEADER environment variable to customize the HTTP Server
  response header, defaulting to Apache/2.2.22 (Ubuntu). This allows the
  honeypot to masquerade as different web servers to attract attackers.

  - Add server_header field to Config dataclass
  - Override version_string() in Handler to return configured header
  - Update documentation and all deployment configs

src/server.py

@@ -31,6 +31,7 @@ def print_usage():
     print('  DASHBOARD_SECRET_PATH - Secret path for dashboard (auto-generated if not set)')
     print('  PROBABILITY_ERROR_CODES - Probability (0-100) to return HTTP error codes (default: 0)')
     print('  CHAR_SPACE            - Characters for random links')
+    print('  SERVER_HEADER         - HTTP Server header for deception (default: Apache/2.2.22 (Ubuntu))')
 
 
 def main():