PUP-1665 Puppet cert list optimization

In our production environment we have >80000 certificates. Issuing ```puppet cert list``` on a large volume of signed certificates "hangs" even though viewing certs does not requiring traversing the signed repo.

It seems looking through the code that puppet does not use/need the list of signed certs when viewing cert requests or individual certs.

```puppet cert list``` should only scan all the signed certs when necessary (signed or all options).