Missed config keys on Scala 2.13.x

The origin of the issue is iteration through the config keys [here](https://github.com/apache/openwhisk/blob/master/common/scala/src/main/scala/org/apache/openwhisk/common/Config.scala#L145) and [here](https://github.com/apache/openwhisk/blob/master/common/scala/src/main/scala/org/apache/openwhisk/common/Config.scala#L156) and at the same time properties [change](https://github.com/apache/openwhisk/blob/master/common/scala/src/main/scala/org/apache/openwhisk/common/Config.scala#L150) in the loop which can lead to skipping some keys after `+=`.

Fixed an issue with missed config keys while iterating through them on Scala 2.13.x
!Note: This issue reproduces only on Scala >= 2.13.x version and probably related to some internal changes in Scala.