Get rid of mess with 'cls' and 'self' in deployment serializer

Based on comments in
https://review.openstack.org/#/c/119033/4/nailgun/nailgun/orchestrator/deployment_serializers.py

Changed 'cls' to 'self'

Related-Bug: #1364532

Change-Id: I33b90431668f3255abb7b1f33949a8f3e61f35fe