This commit adds option ps2mouse to pointer_model, and set the default
value of pointer_model to usbtablet to do not break upgrade regarding
the default behavior of use_usb_tablet.
WHY: use_usb_tablet is by default set to True and during the
deprecation phase of use_usb_tablet, operators which have set that
option to false can't have the same behavior by using pointer_model
since use_usb_tablet takes precedence. Now operators can use
pointer_model=ps2mouse.
Change-Id: Id18b5503799922e4096bde296a9e7bb4f2a994aa
Closes-Bug: #1621582