This patch adds notifications for create, update or delete
port groups. Event types are:
baremetal.portgroup.{create, update, delete}.{start,end,error}.
Developer documentation updated. "portgroup_uuid" field added to
port payload.
Closes-Bug: #1660292
Change-Id: I9a8ce6c34e9c704b1aeeb526babcb20a5b1261db