Xapi supports shadow password files

We're moving XenServer dom0 to CentOS 6.4 (finally) and we need to update Xapi so that it understands shadow passwords. All of the logic for interfacing with the passwd and shadow files was put in a separate library (https://github.com/mcclurmc/ocaml-opasswd), on which Xapi now depends.

A number of things have changed in this patch series:

1. We no longer check config file hashes
2. We don't send the entire passwd file, just the root passwd wrapped in JSON
3. The URI for the config file sync handler is versioned: /sync_config_files/2
4. The original URI still works, to support upgrades
5. The original handler outputs the original passwd file, with the passwords inserted from the shadow file, if it exists.

Don't pull this until the ocaml-opasswd.spec.in file is merged into xen-api-libs-specs (https://github.com/xapi-project/xen-api-libs-specs/pull/19)