roles/kerberos: Configure mit-krb5

The *kerberos* role configures the MIT Kerberos library.  Specifically,
it creates `/etc/krb5.conf` and populates it with some basic default
options. It also creates the `/etc/krb5.conf.d` directory, into which
other roles can write additional configuration files.

roles/kerberos/defaults/main.yml

@@ -0,0 +1,6 @@
+krb5_dns_lookup_realm: false
+krb5_ticket_lifetime: 24h
+krb5_renew_lifetime: 7d
+krb5_forwardable: true
+krb5_rdns: false
+krb5_default_ccache_name: KEYRING:persistent:%{uid}