Allow registry, registry username and password to be empty strings (#254

)

## Description


Allow registry, registry username, and password to be empty strings. These values are passed as kernel command line args for OSIE's boot up and then used by Tink worker when it pulls and runs containers. With [this](tinkerbell/tink#607) change to Tink worker, allowing the empty string here provides Tink worker the ability to specify any fully qualified container image in a template. e.g. `docker.io/alpine`, `quay.io/tinkerbell-actions/cexec`, `public.ecr.aws/l0g8r8j6/tinkerbell/pbnj`, etc

This change is backward compatible and shouldn't negatively affect using the Cacher backend.

## Why is this needed



Fixes: #

## How Has This Been Tested?





## How are existing users impacted? What migration steps/scripts do we need?





## Checklist:

I have:

- [ ] updated the documentation and/or roadmap (if required)
- [ ] added unit or e2e tests
- [ ] provided instructions on how to upgrade

installers/osie/main.go

@@ -114,12 +114,18 @@ func kernelParams(ctx context.Context, action, state string, j job.Job, s ipxe.S
 
 	if j.CanWorkflow() {
 		buildWorkerParams()
-		s.Args("docker_registry=" + dockerRegistry)
+		if dockerRegistry != "" {
+			s.Args("docker_registry=" + dockerRegistry)
+		}
 		s.Args("grpc_authority=" + grpcAuthority)
 		s.Args("grpc_cert_url=" + grpcCertURL)
 		s.Args("instance_id=" + j.InstanceID())
-		s.Args("registry_username=" + registryUsername)
-		s.Args("registry_password=" + registryPassword)
+		if registryUsername != "" {
+			s.Args("registry_username=" + registryUsername)
+		}
+		if registryPassword != "" {
+			s.Args("registry_password=" + registryPassword)
+		}
 		s.Args("packet_base_url=" + workflowBaseURL())
 		s.Args("worker_id=" + j.HardwareID().String())
 	}

installers/osie/mirror.go

@@ -52,11 +52,11 @@ func mustBuildOSIEURL() *url.URL {
 }
 
 func buildWorkerParams() {
-	dockerRegistry = getParam("DOCKER_REGISTRY")
+	dockerRegistry = os.Getenv("DOCKER_REGISTRY")
 	grpcAuthority = getParam("TINKERBELL_GRPC_AUTHORITY")
 	grpcCertURL = getParam("TINKERBELL_CERT_URL")
-	registryUsername = getParam("REGISTRY_USERNAME")
-	registryPassword = getParam("REGISTRY_PASSWORD")
+	registryUsername = os.Getenv("REGISTRY_USERNAME")
+	registryPassword = os.Getenv("REGISTRY_PASSWORD")
 }
 
 func getParam(key string) string {