webadmin: fixed action availability in tree context

When a specific item (for example, an actual host -- not
"Hosts") is selected in the left-pane tree, we need to
hide all flavors of the "new" and "remove" buttons / context
menu items.

Change-Id: I846346e683db7210753c4e85426a210195509835
Bug-Url: https://bugzilla.redhat.com/858952
Signed-off-by: Greg Sheremeta <gshereme@redhat.com>