[client] Name the two config readers for what they do

ReadConfig and GetConfig differed in one thing — what happens when the file is
absent — and neither name said which was which:

- ReadConfig      -> ReadOrGenerateConfig  (reads it, or generates one in memory)
- GetConfig       -> GetExistingConfig     (reads it, or fails)

Three comments went with them:

- GetConfig's said "return with Config and if it was created. Errors out if it
  does not exist", which described a bool it does not return and a creation it
  never performs.
- ReadConfig's explained that it does not write, which is what a reader is
  supposed to do anyway.
- Server.getConfig's said it "errors out if it does not exist", which it does
  not — it resolves a default config, and now provisions the identity too.

client/cmd/login.go

@@ -326,7 +326,7 @@ func doForegroundLogin(ctx context.Context, cmd *cobra.Command, setupKey string,
 
 	}
 
-	config, err := profilemanager.ReadConfig(configFilePath)
+	config, err := profilemanager.ReadOrGenerateConfig(configFilePath)
 	if err != nil {
 		return fmt.Errorf("read config file %s: %v", configFilePath, err)
 	}