support for admin_server option

adding an optional attribute ['krb5']['default_realm_admin_server'] which will allow you to add the "admin_server" tag to the default realm section.  This provides configuration necessary to run kadmin commands against a remote administration server.

example input:
```
      :krb5 => {
        "default_realm" => "example.com",
        "default_realm_admin_server" => "kdc1.example.com",
        "default_realm_kdcs" => [
          "kdc1.example.com",
          "kdc2.example.com",
          "kdc3.example.com"
        ]
      }
```
example output:
```
[realms]
 EXAMPLE.COM = {
  kdc = kdc1.example.com
  kdc = kdc2.example.com
  kdc = kdc3.example.com
  admin_server = kdc1.example.com
 }
```