Remove plugin from service/instances.py

The usage of plugin in savanna.service.instances:scale_cluster is improper.
The instances should work with infrastructure only and do not call plugin
methods.

That is the preliminary change before introducing Heat provisioning.

Implements blueprint infra-provisioning-extensions

Change-Id: I98ba640ae13bf2067c5e985c9009e21dd813b3f3
Merge "Remove plugin from service/instances.py"