Add log shim and collect command

The collect command redirects function logs to the journal for
viewing on journalctl. faas-cli logs is not implemented as of
yet. View logs with journalctl -t openfaas-fn:FN_NAME_HERE.

Tested on Dell XPS with Ubuntu Linux. The approach takes
inspiration from the Stellar project.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"fmt"
 	"os"
 
 	"github.com/openfaas/faasd/cmd"
@@ -15,6 +16,21 @@ var (
 )
 
 func main() {
+
+	if _, ok := os.LookupEnv("CONTAINER_ID"); ok {
+		collect := cmd.RootCommand()
+		collect.SetArgs([]string{"collect"})
+		collect.SilenceUsage = true
+		collect.SilenceErrors = true
+
+		err := collect.Execute()
+		if err != nil {
+			fmt.Fprintf(os.Stderr, err.Error())
+			os.Exit(1)
+		}
+		os.Exit(0)
+	}
+
 	if err := cmd.Execute(Version, GitCommit); err != nil {
 		os.Exit(1)
 	}