Stop using "tkey-gssapi-credential" in tests

Stop using "tkey-gssapi-credential" in tests

Since the "tkey-gssapi-credential" statement is now deprecated and is
about to be removed, migrate the only system test using it ("nsupdate")
to "tkey-gssapi-keytab".

Currently, the GSS-TSIG parts of the "nsupdate" system test require
properly setting up a combination of:

  - "tkey-gssapi-credential" statements in named.conf files,
  - the KRB5_KTNAME environment variable.

Specifically, this configuration causes named startup to include
acquiring the credential that GSS-API is allowed to match keys against
from a keytab file specified by the KRB5_KTNAME environment variable.

By contrast, the revised configuration uses the "tkey-gssapi-keytab"
statement, which makes GSS-API match keys against any credential present
in the specified keytab file.

Since both keytabs in question (ns9/dns.keytab, ns10/dns.keytab) only
contain a single credential, the two configurations are functionally
equivalent, with the revised one being significantly more readable and
simpler to prepare.