TraceServer should fall back to GENERIC_KERBEROS_PRINCIPAL when trace specific kerberos settings are not present

Prior to 1.7, the TraceServer always started using the same server utils as the other daemons. Since a trace server has to talk to Accumulo and that might involve needing a Kerberos Identity in 1.7+, it was switched to its own setup.

Currently that setup will default back to GENERIC_KERBEROS_KEYTAB if a keytab isn't specified for the trace user, but it will simply exit early if there isn't a principal defined for hte trace user. It should instead default to the GENERIC_KERBEROS_PRINCIPAL.