Allow to set URL to Faktory instance via FAKTORY_URL env variable

[Envek]

Duplicated from praekeltfoundation#3

Context

In containerized world (especially in Kubernetes) it is very common pattern to pass secret values via environment variables without exposing them in command line arguments or elsewhere.

What's inside

Ability to specify URL to faktory instance (including password) via FAKTORY_URL environment variable.

Kubernetes usage

Example of defining sidecar with exporter for Faktory server pod (see AdWerx/charts#12 for real example of usage)

# Somewhere deep in StatefulSet declaration
        - name: metrics-exporter
          image: "praekeltfoundation/faktory_exporter:notyetreleased"
          imagePullPolicy: Always
          env:
            - name: FAKTORY_PASSWORD
              valueFrom:
                secretKeyRef:
                  name: faktory
                  key: password
            - name: FAKTORY_URL
              value: tcp://:$(FAKTORY_PASSWORD)@localhost:7419