Expose riak_client:repair_node() via CLI

[martinsumner]

When running Riak with ephemeral disks, the repair_node function is an important recovery process. Currently this is available via remote_console (i.e. riak_client:repair_node()) but isn't part of the console CLI (or indeed documented).

In discussions it has been emphasised the important to some users in having things available via CLI rather than remote_console - and this is a good example of something that should be exposed via the CLI, with some appropriate help guidelines (and documented).