Bug 6856: Rpc definition should implicitly define input/output

Bug 6856: Rpc definition should implicitly define input/output

The yangtools patch for this bug required a minor change in mdsal's
binding generator v1 to maintain compatibility. The mdsal patch has
already been merged. The binding generator v1 now generates classes
for input and output statement only if they are explicitly declared
in the model. As a consequence, a minor adjustement has to be made
in BindingToNormalizedNodeCodec's findRpcMethod().

This patch needs to be merged to unblock the distribution-check job
in the yangtools patch.

Change-Id: I6ed0e57aae35987ae943c8aa7a4a79861af04f7a
Signed-off-by: Igor Foltin <ifoltin@cisco.com>