webadmin: Change checkbox column to radio button column

The semantics when managing cluster networks were those of a radio
button column implemented through a checkbox column, so now the view
is changed to also look like radio buttons. This was done by extending
the capabilities of CheckboxColumn, to also function as a radio button
column when needed.

Change-Id: I6576b816c53e482b54d49877eaa04088aa9f5960
Bug-Url: https://bugzilla.redhat.com/1023722
Signed-off-by: Lior Vernia <lvernia@redhat.com>