Allow tagging of nodes

Instead of arbitrary grouping of nodes, where one node can be part of any group (even multiple groups), tagging concept will be more natural for this feature. User can select whatever tag he wants to use for the node, he doesn't have to create tags in advance, and already existing tags will be suggested for him when he is tagging nodes.

Then, for monitoring or any other purposes (bulk actions), he can very easily filter based on the tags.