Replace ca_certificate_bundle by ca_certificate
Replace ca_certificate_bundle by ca_certificate

Some parts of the doc still mention ca_certificate_bundle.pem whereas the rest refers to ca_certificate.pem.
Updated all references to ca_certificate_bundle.pem to make it coherent.
rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website