Add support for Scientific Linux as a centos target

```
(greg.poirier@oppie)(~/dev/ceph-deploy)(scientific_support)
ceph-deploy install test-ceph-1001 test-ceph-1002 test-ceph-1003

[root@test-ceph-1001 ~]# yum list ceph
Loaded plugins: security
Installed Packages
ceph.x86_64                                                    0.61.2-0.el6                                                     @ceph
[root@test-ceph-1001 ~]# which ceph
/usr/bin/ceph
```

I promise uninstall works too.