Add possibility to get list of datacenters in rune scripts

[vponomaryov]

It is useful for having better flexibility running a test against a multi-dc DB clusters.

Also, share preferred_datacenter info with rune scripts.

[pkolaczk]

Can you explain why it is useful?

And if we add that, I think we also need to expose other metadata, e.g. list of nodes, cluster name, versions, etc.
Anyways, instead of adding it to the top-level context object, maybe let's have something like connection field which would expose the metadata of the connection (and maybe also offer some less frequently used options)? I don't want to pollute the namespace of the top-level object with things that aren't frequently used.

[pkolaczk]

As commented above, let's introduce a connection field on context, and let's add all the connection stuff there. This is because context is not just about the connection, its responsibility is bigger, e.g. storing temporary data (not related to the database) between prepare and run step.

[vponomaryov]

Can you explain why it is useful?

We have a multi-dc setup where region-specific stuff gets used in schema.
Then, a stress that gets run must be aware about the preferred datacenter if it is set for a latte command.