dogtaginstance: avoid special cases for Server-Cert
The Dogtag "Server-Cert cert-pki-ca" certificate is treated
specially, with its own track_servercert() method and other special
casing. But there is no real need for this - the only (potential)
difference is the token name. Account for the token name difference
with a lookup method and treat all Dogtag system certs equally
w.r.t. tracking request creation and removal.
Part of: https://pagure.io/freeipa/issue/7991
Reviewed-By: Rob Crittenden <rcritten@redhat.com>