Provide overrides for proxy.properties in Docker deployments

@keith-turner and I briefly touched upon allowing `proxy.properties` to be overidden for a Docker instantiation of accumulo-proxy in this pull request: https://github.com/apache/accumulo-proxy/pull/20

Currently in accumulo-docker you can override properties by the following:
```
export ACCUMULO_CL_OPTS="-o prop.1=var1 -o prop.2=var2"
docker run -d --network="host" accumulo monitor $ACCUMULO_CL_OPTS
```
This is documented in the docs here: https://github.com/apache/accumulo-docker

For standard docker approaches I would say this is a simplistic mechanism that works really cleanly however if you start to look at things like docker-compose or kubernetes I would say it would be advantageous to avoid overriding the CMD or ENTRYPOINT flags where possible.

An alternative method would be to provide these as environment variables, therefore you could also use the Kubernetes secret APIs to handle things such as passwords and either map them to a file or environment variable for use within the application.

Would this be acceptable? If so I'm also happy to look at providing the same change to accumulo-docker to retain consistency?