Add sssd and pam_unix customization

Sorry for this being such a large pull request. For RHEL, they recommend the use of [sssd](https://fedorahosted.org/sssd/) to cache security creds to limit load on your ldap server so I have added this functionality since we needed it. On top of that, we use sha512 instead of md5 in our pam_unix configuration so I added the ability to customize this the same way you have the others. The default is still md5 though. Let me know if you have any further questions or requests. Thanks.