certread-n: Look up certs using PK11_FindCertFromNickname()
Don't loop through all the tokens looking for a given nickname, look
it up directly. If a nickname does not contain a token then NSS
treats it as the internal token. Otherwise it uses the token name
to retrieve the slot and looks for certificates in that slot.
Looping through the certificates in each slot using
PK11_ListCertsInSlot() was sometimes taking as many as 14 seconds
for no apparent reason. This slowdown is not seen when using
PK11_FindCertFromNickname().
This 14 second delay was causing client DBus timeouts which was
causing IPA server installation failures when running start-tracking
on the CA subsystem certificates.
Related IPA issue https://pagure.io/freeipa/issue/8533