CAS-1326: The logout does not work : no back channel communication, nor ...

...front channel redirections

In the **TerminateSessionAction** class :
For logout, the way the TGT is retrieved, does not work :
*String tgtId = WebUtils.getTicketGrantingTicketId(context);* does not return anything.
The TGT's value must be found through the cookie's value :
*String tgtId = this.ticketGrantingTicketCookieGenerator.retrieveCookieValue(request);*