nis: add MIT kerberos package

Lysator uses MIT kerberos and GSSAPI authentication with SSH won't work
unless the MIT version is installed.

manifests/nis.pp

 class freebsd::nis {
+  package {
+    [
+      'krb5',
+    ]:
+      ensure => installed,
+  }
+
   file { '/etc/auto_master':
     ensure => file,
     source => 'puppet:///modules/freebsd/nis/auto_master',