Add a service sla lookup function

- ITACLOUD-2811
Refactor service view implementation
Implement a skeleton of regular SLA value update
Extend regular sla update tasks
Merge branch 'develop' of code.opennodecloud.com:nodeconductor/nodeconductor into feature/itacloud-2811

Conflicts:
	nodeconductor/iaas/views.py
Fix deprecation warnings
Allow InstanceSlaHistory to be managed by staff
Fix period format and add docs
Merge branch 'feature/itacloud-2811' into 'develop'

Feature/itacloud 2811 - sla sync