Move constants to config, update string formatting

Not sure if this is the Elixir-y way to do this, but seems more logical
than hardcoding values such as redis connection.

Also went through and improved how string formatting was performed
throughout the app. Rather than "combining" <> "strings" this way, I'm
now just doing "#${variable}#{formatting}", which looks a lot cleaner.

update.exs

@@ -8,6 +8,15 @@ defmodule Service do
 end
 
 defmodule Instances do
+  @fallback_str Application.fetch_env!(:privacy_revolver, :fallback_str)
+  @update_file Application.fetch_env!(:privacy_revolver, :update_file)
+  @services_json Application.fetch_env!(:privacy_revolver, :services_json)
+
+  def init() do
+    File.rename(@update_file, "#{@update_file}-prev")
+    update(@services_json)
+  end
+
   def request(url) do
     case HTTPoison.get(url) do
       {:ok, %HTTPoison.Response{status_code: 200}} ->
@@ -19,10 +28,6 @@ defmodule Instances do
   end
 
   def update(filename) do
-    {:ok, conn} = Redix.start_link(
-      "redis://localhost:6379",
-      name: :redix
-    )
     {:ok, file} = File.read(filename)
     {:ok, json} = Poison.decode(file, as: [%Service{}])
 
@@ -32,20 +37,20 @@ defmodule Instances do
         request(instance_url <> service.test_url) == :good
       end)
 
-      add_to_redis(conn, service, result)
+      add_to_redis(service, result)
       log_results(service.type, result)
     end
   end
 
-  def add_to_redis(conn, service, instances) do
+  def add_to_redis(service, instances) do
     # Remove previous list of instances
-    Redix.command(conn, [
+    Redix.command(:redix, [
       "DEL",
       service.type
     ])
 
     # Update with new list of available instances
-    Redix.command(conn, [
+    Redix.command(:redix, [
       "LPUSH",
       service.type
     ] ++ instances)
@@ -53,26 +58,25 @@ defmodule Instances do
     # Set fallback to one of the available instances,
     # or the default instance if all are "down"
     if Enum.count(instances) > 0 do
-      Redix.command(conn, [
+      Redix.command(:redix, [
         "SET",
-        service.type <> "-fallback",
+        "#{service.type}#{@fallback_str}",
         Enum.random(instances)
       ])
     else
-      Redix.command(conn, [
+      Redix.command(:redix, [
         "SET",
-        service.type <> "-fallback",
+        "#{service.type}#{@fallback_str}",
         service.fallback
       ])
     end
   end
 
   def log_results(service_name, results) do
-    {:ok, file} = File.open(".update-results", [:append, {:delayed_write, 100, 20}])
-    IO.write(file, service_name <> ": " <> inspect(results) <> "\n")
+    {:ok, file} = File.open(@update_file, [:append, {:delayed_write, 100, 20}])
+    IO.write(file, "#{service_name}: #{inspect(results)}\n")
     File.close(file)
   end
 end
 
-File.rename(".update-results", ".update-results-prev")
-Instances.update("services.json")
+Instances.init()