Added docs about new snmp related resource types
Added docs about new snmp related resource types

Added docs about the newly added snmp related resource types: host,
host_disk, host_network_interface.

Change-Id: I3f638fc4332a992d7238444c1b41bcb00268d047

sileht/gnocchi,sileht/gnocchi,leandroreox/gnocchi,gnocchixyz/gnocchi,gnocchixyz/gnocchi,leandroreox/gnocchi