`crypto/x509/pkix.Name` now implements `String() string`, use it

[philpennock]

In the code, wherever we currently reference .CommonName, that was a short-cut because I didn't want to write a full name construction function.

As of Go 1.10, per release notes:

crypto/x509/pkix
Name now implements a String method that formats the X.509 distinguished name in the standard RFC 2253 format.

We should switch to using this and make sure the results still look sane.