In case you are using a ssh-keygen public key to connect to your server/VPS, and want to use it to start a server setup using ClusterCS, you will need to convert to a PEM format.
First, make sure you have a file that contains only your key, let's say it's called server.pub. Afterwards run the following command:
ssh-keygen -f server.pub -e -m pem
In the terminal you will see the key in PEM format. Use that to start your setup on ClusterCS!