Fixes #123: Add option to prune deleted clients

Added an option (--prune-clients) that allows to prune clients that have
been deleted from Chef when rotating vault keys.  This option is
available for the knife vault remove|rotate all keys|rotate keys
commands.  This has been implemented by adding an optional prune
argument to the rotate_keys and rotate_all_keys functions (false by
default).

Also added aruba CLI tests to test the option in combination with each
of the three commands.