providers/os: Add ability to read environment variables

providers/os/os.go

@@ -27,6 +27,7 @@ type Provider struct {
 // FuncMap fulfills the step.FuncProvider interface.
 func (this *Provider) FuncMap () template.FuncMap {
 	return template.FuncMap {
+		"env":        funcEnv,
 		"fileExists": funcFileExists,
 		"statFile":   funcStatFile,
 		"readFile":   funcReadFile,
@@ -37,6 +38,10 @@ func (this *Provider) FuncMap () template.FuncMap {
 	}
 }
 
+func funcEnv (name string) string {
+	return os.Getenv(name)
+}
+
 func funcFileExists (name string) (bool) {
 	_, err := os.Stat(name)
 	return err == nil