cartridges/README.md: add link to broker-node doc

Add a link to documentation/communication-between-broker-and-node.md
in the description of the cartridge-list action.
docs: describe the cartridge-list action

In documentation/communication-between-broker-and-node.md, describe the
cartridge-list action of the cartridge_do request to the mcollective agent.
node/README.md: Add a description of the node
cartridges/README.md: *-port hooks are deprecated

Document that expose-port, show-port, and conceal-port are deprecated.

While we're at it, reformat the list of hooks that still need to be
documented.