RAPI: implement instance reinstall

This patch adds instance reinstall to RAPI, with two optional parameters:
  - ‘os', in order to change the OS on reinstall
  - ‘nostartup’, in order to leave the instance down after reinstall

The call will first shutdown the instance, the reinstall it, and unless
‘nostartup’ has been passed and is equal to 1, it will be started
automatically.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>