rpk:make redpanda start use loaded file or default

In the viper removal PR (redpanda-data#5061) we introduced
a bug:

rpk redpanda start will load the file and update
some unset defaults such as rpk.kafka_api.

This breaks backward compatibility for some users
so we are correcting it in this commit.

src/go/rpk/pkg/cli/cmd/redpanda/start.go

@@ -152,6 +152,7 @@ func NewStartCommand(fs afero.Fs, launcher rp.Launcher) *cobra.Command {
 			if err != nil {
 				return fmt.Errorf("unable to load config file: %s", err)
 			}
+			cfg = cfg.FileOrDefaults()
 
 			if len(configKvs) > 0 {
 				if err = setConfig(cfg, configKvs); err != nil {

src/go/rpk/pkg/cli/cmd/redpanda/start_test.go

@@ -14,9 +14,7 @@ package redpanda
 
 import (
 	"bytes"
-	"net"
 	"os"
-	"strconv"
 	"testing"
 
 	"github.com/redpanda-data/redpanda/src/go/rpk/pkg/config"
@@ -194,10 +192,6 @@ func TestStartCommand(t *testing.T) {
 				path,
 			)
 			c := config.Default()
-			// Adding unset default that get added on first load.
-			b0 := c.Redpanda.KafkaAPI[0]
-			c.Rpk.KafkaAPI.Brokers = []string{net.JoinHostPort(b0.Address, strconv.Itoa(b0.Port))}
-			c.Rpk.AdminAPI.Addresses = []string{"127.0.0.1:9644"}
 
 			conf, err := new(config.Params).Load(fs)
 			require.NoError(st, err)

tests/rptest/tests/rpk_config_test.py

@@ -53,15 +53,9 @@ def test_config_init(self):
           port: 9644
     developer_mode: true
 rpk:
-    admin_api:
-        addresses:
-            - 127.0.0.1:9644
     coredump_dir: /var/lib/redpanda/coredump
     enable_memory_locking: false
     enable_usage_stats: false
-    kafka_api:
-        brokers:
-            - 0.0.0.0:9092    
     overprovisioned: false
     tune_aio_events: false
     tune_ballast_file: false
@@ -182,9 +176,6 @@ def test_config_set_json(self):
         rpk.config_set(key, value, format='json')
 
         expected_config = yaml.full_load('''
-admin_api:
-    addresses:
-        - 127.0.0.1:9644
 coredump_dir: /var/lib/redpanda/coredump
 enable_memory_locking: false
 enable_usage_stats: false  
@@ -210,12 +201,6 @@ def test_config_set_json(self):
             with open(os.path.join(d, 'redpanda.yaml')) as f:
                 actual_config = yaml.full_load(f.read())
 
-                assert actual_config['rpk']['kafka_api'] is not None
-
-                # Delete 'kafka_api' so they can be compared since the
-                # brokers change depending on the container it's running
-                del actual_config['rpk']['kafka_api']
-
                 if actual_config['rpk'] != expected_config:
                     self.logger.error("Configs differ")
                     self.logger.error(