worker/certupdater: add "juju-apiserver" as DNS name

The in-tree API clients are using "juju-apiserver" as the
hostname to verify when connecting to the API server,
rather than the actual hostname. The certupdater worker
was not setting this as a DNS name in the certificates it
generates. The Azure provider doesn't record any public IP
addresses, so the verification always takes place and always
fails.

We fix this by adding "juju-apiserver" as a DNS name in the
certificates. Ideally the clients should be using the actual
hostname used to connect with, but this will break backwards-
compatibility with older versions of Juju.

Fixes https://bugs.launchpad.net/juju-core/+bug/1400348