fixes #6511 / BZ 1109085 - content host bulk errata fixes

This commit addresses a couple of small UI issues with the
content host bulk errata action.  The issues were:

1. If no content hosts are selected, do not retrieve applicable
   errata from the server.  The API requires the user to provide
   at least 1 content host; therefore, no need to send the request
   with 0.
2. If no content hosts are selected, disable the refresh and
   install actions.
Better to use the pass the data being watched that is passe dinto the function here. For example:

```
$scope.$watch('nutupane.table.numSelected', function (numSelected) {
    if ((numSelected > 0) ...
});
```