Add warning massage about low disk space on /var/log partition on fuel master node

Please add warning massage about low disk space on /var/log partition on fuel master node.

It will help Customers to free space to prevent 100% partition space usage.

100% partition space usage can be reached if Customer create and delete a lot of test environments using the same fuel master node. So in case if it is the same fuel master where production cloud(s) is(are) running Customers can reach the problem with docker containers in case of no free space on /var/log partition.

Another solution in this case - remove logs from fuel master node if cluster does not exist any more for XX days, and remove logs from bootstraped nodes older that XX days.