as per documentation i have created 1st config file
juju generate-config -w command.
Do I need to create separate" .yaml" file for ceph or openstack services? or i can just add new environment to existing config file?
You do not need to add new files, you can just add new environments in the same file. The example generates a section for openstack already, you can just edit it and uncomment the sections that make sense for your cloud.