This CR adds an opaque section 'Consensus' in orderer.yaml to
contain arbitrary config options needed by consensus plugin.

Consensus plugin should use viperutil.Decode to decode the
content of this section to a predefined struct.

In this way, we don't need to expand TopLevel config for each
consensus plugin, therefore maximizing flexibility.

Change-Id: Ie657f1ca69cc79a8aaaf629d2df1bd0164cf5164
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>