Implements proper permissions on airship2 generated files

*This commit sets default permissions for .airship directory & config
  file.
*It allows operators to override default permissions in config file

Closes: #188

Change-Id: I5713662ecc71b71c0d0bd0f61691d326c7727b0f

testutil/testconfig.go

@@ -42,6 +42,10 @@ func DummyConfig() *config.Config {
 		AuthInfos: map[string]*config.AuthInfo{
 			"dummy_user": DummyAuthInfo(),
 		},
+		Permissions: config.Permissions{
+			DirectoryPermission: config.AirshipDefaultDirectoryPermission,
+			FilePermission:      config.AirshipDefaultFilePermission,
+		},
 		BootstrapInfo: map[string]*config.Bootstrap{
 			"dummy_bootstrap_config": DummyBootstrapInfo(),
 		},