Send "501 Not Implemented" back when method not found
Send "501 Not Implemented" back when method not found

Before this was "400 Bad Request" and thus it didn't reflect
the reality.

Signed-off-by: René Nussbaumer <bc755ac45a0e799d1d490cba76febc56d80cc5fc@google.com>
Reviewed-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>

ganeti/ganeti,mbakke/ganeti,leshchevds/ganeti,onponomarev/ganeti,andir/ganeti,bitemyapp/ganeti,leshchevds/ganeti,grnet/snf-ganeti,apyrgio/snf-ganeti,ganeti/ganeti,andir/ganeti,mbakke/ganeti,dimara/ganeti,yiannist/ganeti,leshchevds/ganeti,onponomarev/ganeti,mbakke/ganeti,dimara/ganeti,bitemyapp/ganeti,grnet/snf-ganeti,apyrgio/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,yiannist/ganeti,apyrgio/snf-ganeti,apyrgio/ganeti,andir/ganeti