Try to detect existing SSL certs before regenerating them

[micahjsmith]

LetsEncrypt limits the number of certificates that can be consecutively generated for the same domain. If full integration tests, that generated SSL certs, are repeatedly run (or if the entire build process is run for some other reason), then this causes an error. As a temporary fix, we break out the routine from make build to a separate make ssl, but a better approach would be to check if certificates exist in the secrets volume before trying to regenerate them.