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.
This is a best practice because you can cause an infinite loop by modifying the data on the `$scope` during a `$watch`.