convert privateCert to PEM for signing

[warncke]

allow privateCert to be passed in as base64 string without PEM formatting the same as cert.

[markstos]

I might use this myself. Please amend your commit to update the docs as well and I think I'll merge this.

The version of systemd on Ubuntu 18.04 doesn't like newlines in environment variables, so this will be a welcome update for those who want to store the PEM in a one-line environment variable.

[cjbarth]

@warncke I second @markstos comments. Please update the docs and lets get this landed.

[warncke]

@markstos @cjbarth sorry for the long delay on this. I have update the readme to try to accurately reflect the change.