eval SEEDS when it contains ENV reference

In case we link containers using environment reference, `SEEDS` variable needs to eval that reference prior to escaping.
This usually happens when the orchestration for docker uses escaping for env variables (like Kubernetes does)

e.g.

`docker run -e SEEDS="\${INFLUXDB_MASTER_HOST}:\${INFLUXDB_MASTER_PORT}" tutum/influxdb`
before the patch: `SEEDS=${INFLUXDB_MASTER_HOST}:${INFLUXDB_MASTER_PORT}` as raw text
after the patch: `SEEDS=10.2.3.10:8090`