Feature/distributed metadata

[kosticmarin]

Resolves #121

This PR adds a new argument when starting nodes called tag which can repeat. The tag argument is used to store additional user defined metadata about the node in a form of a key=value, e.g. --tag name=node01.

Further we add a two new functions for looking up nodes via a query. The query syntax is like URL query string. The first function exec_lookup_nodes executes the query on the control node and stores the results in the control client, returning a query id and query results size. The second function copy_lookup_nodes_results copies the query results to the guest memory based on the query id.

Finally on the control server we add a parser for the query string.