properly split ssh key ids

This pull request is to adjust the way multiple ssh_keys_ids are to be split into a ruby list. I adjusted the code to match what the README says - assuming the README was the way this feature should work.

Without this PR, multiple ssh keys should be defined like so:

DIGITALOCEAN_SSH_KEY_IDS = "1234 4321"

After this PR it will do the right thing for both of these situations:

DIGITALOCEAN_SSH_KEY_IDS = "1234,4321"
DIGITALOCEAN_SSH_KEY_IDS = "1234, 4321"