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
Merge pull request #16 from bivas/patch-1

eval SEEDS when it contains ENV reference