Don't obtain cert if one already exists (persist LE User)

If I spawn a LE stack, it gets a certificate and stores it in the certificate store. If I then remove the stack and recreate it with the same settings, it will 'Found existing certificate 'cake' in Rancher' but still go out and grab a fresh one from LetsEncrypt.

For production usage, this isn't safe. I never want to pull a new certificate unnecessarily unless the previous one is about to expire (or maybe if I enable that functionality and in sandbox mode). If it's a worry about populating the local volume/directory with the certificate, we should pull the present one from the Certificate Store and save it.