If channel creation config contains only a subset of orderers
in system channel, it should just create a new channel with
specified set of orderers, instead of reconfiguring system
channel. This CR fixes config block commitment in etcdraft
to properly handle channel creation config.

Change-Id: I408e707ae580ce041e8fc98e76311226437720ae
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>