Retire DISABLE_CLOUD_CONFIG variable

[mkulke]

All the user-data config is cloud-config formatted now, we should fix all mentions and code paths:

$ git grep DISABLE_CLOUD_CONFIG
src/cloud-api-adaptor/aws/image/rhel/aws-rhel.pkr.hcl:      "DISABLE_CLOUD_CONFIG=${var.disable_cloud_config}",
src/cloud-api-adaptor/aws/image/rhel/variables.pkr.hcl:  default = env("DISABLE_CLOUD_CONFIG")
src/cloud-api-adaptor/aws/image/ubuntu/aws-ubuntu.pkr.hcl:      "DISABLE_CLOUD_CONFIG=${var.disable_cloud_config}",
src/cloud-api-adaptor/aws/image/ubuntu/variables.pkr.hcl:  default = env("DISABLE_CLOUD_CONFIG")
src/cloud-api-adaptor/azure/build-image.md:> If you want to disable cloud-init then `export DISABLE_CLOUD_CONFIG=true` before building the image.
src/cloud-api-adaptor/azure/image/rhel/azure-rhel.pkr.hcl:      "DISABLE_CLOUD_CONFIG=${var.disable_cloud_config}",
src/cloud-api-adaptor/azure/image/rhel/variables.pkr.hcl:  default = env("DISABLE_CLOUD_CONFIG")
src/cloud-api-adaptor/azure/image/ubuntu/azure-ubuntu.pkr.hcl:      "DISABLE_CLOUD_CONFIG=${var.disable_cloud_config}",
src/cloud-api-adaptor/azure/image/ubuntu/variables.pkr.hcl:  default = env("DISABLE_CLOUD_CONFIG")
src/cloud-api-adaptor/podvm-mkosi/Dockerfile.podvm:RUN PODVM_DISTRO=ubuntu CLOUD_PROVIDER=generic DISABLE_CLOUD_CONFIG=true bash ./misc-settings.sh
src/cloud-api-adaptor/podvm/qcow2/misc-settings.sh:# If DISABLE_CLOUD_CONFIG is not set or not set to true, then add cloud-init.target as a dependency for process-user-data.service
src/cloud-api-adaptor/podvm/qcow2/misc-settings.sh:if [ -z "$DISABLE_CLOUD_CONFIG" ] || [ "$DISABLE_CLOUD_CONFIG" != "true" ]
src/cloud-api-adaptor/podvm/qcow2/rhel/qemu-rhel.pkr.hcl:      "DISABLE_CLOUD_CONFIG=${var.disable_cloud_config}"
src/cloud-api-adaptor/podvm/qcow2/rhel/variables.pkr.hcl:  default = env("DISABLE_CLOUD_CONFIG")
src/cloud-api-adaptor/podvm/qcow2/ubuntu/qemu-ubuntu.pkr.hcl:      "DISABLE_CLOUD_CONFIG=${var.disable_cloud_config}"
src/cloud-api-adaptor/podvm/qcow2/ubuntu/variables.pkr.hcl:  default = env("DISABLE_CLOUD_CONFIG")