RAPI server: Drop support for instance creation format 0

Ganeti 2.1.3, released in June 2010, added support for a new, extensible
instance creation request format, called version 1. This patch removes
support for the old and undocumented version 0 format.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>