test/certs/generate.sh: should regenerate when new service names are added

[jsha]

After a recent pull, I was getting errors running the integration tests because test/certs/ipki/sfe.boulder/ didn't exist. That's because sfe.boulder was recently added to the list of services, but generate.sh doesn't regenerate the certificates if the ipki directory already exists: https://github.com/letsencrypt/boulder/blob/161ca1ed7843c2f4f1205dfa107f75f6abc276fe/test/certs/generate.sh#L70-L74

This error will affect anyone running Boulder, any time we add services to the internal PKI. We should improve the script so it automatically adds new services; for instance we could make the script check the existence of each service name's directory, rather than ipki overall.

[jprenken]

Bump; I ran into this today.