Revert "rapi.client.ModifyNode should PUT rather than POST"

This was a mistake on my side because ModifyGroup and ModifyInstance
were PUT, and I was not aware of the discussion and the rationale why
this one had to be POST.

This reverts commit 55ef0cf6.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>