ncm-opennebula: add support for OpenNebula

This PR fixes these issues:
 * #348 Use TextRender
 * #349 Use Readonly instead of constants
 * #353 Buggy hyp hosts should be included as disabled 

Watch out: the final `else` is not reachable, since both 1 and -1 evaluate to true. You mean:

```perl
if (!$quattor) {
...
} elsif ($quattor == 1) {
...
} else { # This implies $quattor == -1
...
}
```