webadmin: Preselect current CD in ChangeCD dialog

Utilize the new vmDynamic#currentCd field and show the user what CD is
currently inserted in the ChangeCD dialog.

Change-Id: Iff90a0ccd8119c3db36cd2eecbfe0fa82ad0c051
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1025376
Signed-off-by: Martin Betak <mbetak@redhat.com>