Honor etcd legacy v/s new config settings properly (#8510)

This PR also fixes issues related to

- Add proper newline for `mc admin config get` output
  for more than one targets
- Fixes issue of temporary user credentials to have
  consistent output
- Fixes a crash when setting a key with empty values
- Fixes a parsing issue with `mc admin config history`
- Fixes gateway ENV handling for etcd server and gateway

cmd/config/config.go

@@ -405,6 +405,9 @@ func (c Config) SetKVS(s string, defaultKVS map[string]KVS) error {
 			kvs[prevK] = strings.Join([]string{kvs[prevK], sanitizeValue(kv[0])}, KvSpaceSeparator)
 			continue
 		}
+		if len(kv) == 1 {
+			return Error(fmt.Sprintf("key '%s', cannot have empty value", kv[0]))
+		}
 		prevK = kv[0]
 		kvs[kv[0]] = sanitizeValue(kv[1])
 	}