force instance name from config

[jacum]

This little change allows forcing instance name (in fact it should be numeric due to other logics in plugin that I didn't dare to touch). Then the network name can be known in advance.

The following scenario is supported by this change:

• Jenkinsfile using docker 'agent' to build the application and to run the end-to-end tests
• Agent container and all compose containers have to share the same docker network
• Docker network needs to be created in the very beginning, before agent container starts
• In current implementation, sbt-docker-compose plugin creates random networks