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/tasks/main.yml

@@ -0,0 +1,9 @@
+- name: ensure main kerberos configuration is set
+  template:
+    src=krb5.conf.j2
+    dest=/etc/krb5.conf
+- name: ensure kerberos configuration drop-in directory exists
+  file:
+    path=/etc/krb5.conf.d
+    mode=0755
+    state=directory