[CO-390] Create a 'demo' executable for cardano-sl-wallet-new

This code makes use of the 'Cardano.Wallet.Demo' module and show
how it can be used to spawn a demo cluster of nodes.
More details in the README.md

cluster/cardano-sl-cluster.cabal

@@ -60,3 +60,23 @@ library
   exposed-modules: Cardano.Cluster
                    Cardano.Cluster.Environment
                    Cardano.Cluster.Util
+
+
+executable cardano-sl-demo
+  ghc-options:        -threaded -O2 -rtsopts
+  default-language:   Haskell2010
+  default-extensions: DeriveGeneric
+                      LambdaCase
+                      NoImplicitPrelude
+                      OverloadedStrings
+                      TupleSections
+                      TypeApplications
+                      ScopedTypeVariables
+
+  hs-source-dirs:     demo
+  main-is:            Main.hs
+
+  build-depends:      base
+                    , async
+                    , cardano-sl-cluster
+                    , universum

cluster/demo/Main.hs

@@ -0,0 +1,51 @@
+{-| Demo cluster of wallet nodes. See cluster/README.md -}
+
+module Main where
+
+import           Universum
+
+import           Control.Concurrent.Async (waitAny)
+import           System.IO (BufferMode (..), hSetBuffering, stdout)
+
+import           Cardano.Cluster (MaxWaitingTime (..), NodeName (..),
+                     NodeType (..), RunningNode (..), startCluster,
+                     waitForNode)
+
+
+-- | Cluster configuration can be tweaked via ENV vars.
+-- Each ENV var is prefixed with the following.
+--
+-- (e.g. `DEMO_NO_CLIENT_AUTH=True`)
+prefix :: String
+prefix = "DEMO_"
+
+
+main :: IO ()
+main = void $ do
+    hSetBuffering stdout NoBuffering -- Instead of LineBuffering
+
+    putTextLn "Cluster is starting..."
+    cluster <- startCluster prefix
+        [ ("core0", NodeCore)
+        , ("core1", NodeCore)
+        , ("core2", NodeCore)
+        , ("core3", NodeCore)
+        , ("relay", NodeRelay)
+        , ("wallet", NodeEdge)
+        ]
+    handles <- forM cluster $ \case
+        RunningCoreNode (NodeName nodeId) handle -> do
+            putTextLn $ "..." <> nodeId <> " has no health-check API."
+            return handle
+
+        RunningRelayNode (NodeName nodeId) handle -> do
+            putTextLn $ "..." <> nodeId <> " has no health-check API."
+            return handle
+
+        RunningWalletNode (NodeName nodeId) client handle -> do
+            putText "..." >> waitForNode client (MaxWaitingTime 30)
+            putTextLn $ nodeId <> " OK!"
+            return handle
+    putTextLn "Cluster is (probably) ready!"
+
+    waitAny handles