Add DhcpHost v2 section
Add DhcpHost v2 section

DhcpHost v2 was missing, and extraDhcpOptions field was hidden
to the users.

Change-Id: I831d0c3ee5135a21c336c9ab601bd3c422e45780
Fixes: MND-42

midonet/midonet-docs,midonet/midonet-docs,midonet/midonet-docs,midonet/midonet-docs