ncm-opennebula: bug fix enable/disable hyp

Several bug fixes #353 #349:
 * Included a new function to enable/disable hyp host
 * use constant replaced by readonly
 * Documentation changes
so this bock should be 
```perl
my $hostinstance = $one->get_hosts(qr{^$host$}
...
if($hostinstance) {
    $hostinstance->disable();
    $self->info("Disabled existing host $host");
}
...