Enable Apache's headers module

Since adding CORS headers in

  aacac335bfca241e484a27bee6bcb8ae199e3dc5

apache depends on the headers module being present. But it seems this
module's presence was not enforced by puppet. So we enforce it now.

Change-Id: I60045e5df64a9d1278099fbc8beb4a7f364f0635