Move high stratum real CMOs configuration to an overridable attribute

Made this change due to an Ohai bug I came across with Fedora 19/20 hosts misreporting their virtualization status.

ohai virtualization on Fedora 19 reported:
```
mjuarez@foo ~$ ohai virtualization
{
  "system": "lxc",
  "role": "host"
}
```

Whereas ohai virtualization on Ubuntu 14.04 would report:
```
ubuntu@foo:~$ ohai virtualization
[
  [
    "system",
    "xen"
  ],
  [
    "role",
    "guest"
  ]
]
```

All of these hosts were within EC2 and I definitely didn't want to use high stratum.

This pull request moves the logic from template to the attributes to allow it to be overridden if so desired.