Common drop-down for instance specific actions

* horizon/static/horizon/js/horizon.forms.js
  + On document load, remove the instance-specific-actions column
and just show a disabled button group at the top of the table.

* horizon/static/horizon/js/horizon.tables.js
  + When a particular instance check box is selected,
construct the button group and display it
  + When none or more-than-one check box is selected,
just show a disabled button group