YAML entity files
YAML files are the default expected format, so you can use .yml, .yaml, or even .txt - as long as the file contains a valid yaml LIST, then it will be loaded.
/etc/OliveTin/servers.yaml
server1:
state: started
hostname: server1.example.com
ip: 192.168.0.1
server2:
state: started
hostname: server2.example.com
ip: 192.168.0.2
server3:
state: stopped
hostname: server3.example.com
ip: 192.168.0.3