ipa-kdb: override krb5.conf when testing KDC code in cmocka
    
    When testing KDC code in cmocka we rely on libkrb5 defaults.
    libkrb5 would read /etc/krb5.conf by default and would load a KDB
    module from there if it is defined for the test realm (EXAMPLE.COM).
    
    Since EXAMPLE.COM is a common name used for test realms, make sure to
    not using /etc/krb5.conf from the system. Instead, force KRB5_CONFIG to
    /dev/null so that only libkrb5 compiled-in defaults are in use.
    
    In such setup libkrb5 will attempt to load KDB driver db2 for our test
    realm. db2 driver doesn't fail if its database is not available (unlike
    FreeIPA's one), so it survives initialization.
    
    As result, ipa-kdb-tests pass without unexpected breakage.
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>