Don't fail if shared_ca directory exist

voxpupuli#191 ensures
the directory exists but when we use shared_ca, that means we will
declare multiple servers with the same ca. So ensures the directory
exists without failing for redeclaration.

manifests/server.pp

@@ -567,17 +567,17 @@
     group => $group_to_set,
   }
 
-  file { "${etc_directory}/openvpn/${name}":
+  ensure_resource(file, "${etc_directory}/openvpn/${name}", {
     ensure => directory,
     mode   => '0750',
     notify => $lnotify,
-  }
+  })
   if $shared_ca {
-    file { "${etc_directory}/openvpn/${ca_name}":
+    ensure_resource(file, "${etc_directory}/openvpn/${ca_name}", {
       ensure => directory,
       mode   => '0750',
       notify => $lnotify,
-    }
+    })
   }
 
   if $extca_enabled {