be able to configure and test dalton_agent without rebuilding agent images

Currently, any changes to dalton-agent.conf or dalton-agent.py require rebuilding the agent containers.

It would be nice if we could mount the dalton-agent.py and dalton-agent.conf into the container.

One tricky area is this line in the suricata Dockerfile:

```
RUN sed -i 's/REPLACE_AT_DOCKER_BUILD-VERSION/'"${SURI_VERSION}"'/' /opt/dalton-agent/dalton-agent.conf
```

