Fix auto-renew and ls after dokku 0.6.x upgrade

[sseemayer]

letsencrypt:auto-renew and letsencrypt:ls are broken after #64 changed the way Let's Encrypt certificates are installed to be more compatible with dokku.

The problem is the detection of let's encrypt-secured apps by checking for the server certificate being a symlink into the let's encrypt folder.

This check needs to be replaced with something else since the certificate is now a copy of the certificates, not a symlink. Suggestion would be to use openssl to check for the certificate issuer instead (should support both real LE and the fake CA for testing)

[josegonzalez]

You could check that the contents are the exact same?

[sseemayer]

Guess that should work, too.

Edit: Have some real-life responsibilities keeping me from fixing this myself right now but will probably duck back later.