* A Cluster object stores information about the cluster in three ways: * - Segments is basically equivalent to gp_segment_configuration, a plain * list of segment information, and is ordered by content id.

This type only exists to allow us to mock Execute[...]Command functions for testing.

* A RemoteOutput is used to make it easier to identify the success or failure * of a cluster command and to display the results to the user.

* A ShellCommand stores a command to be executed (in both executable and * display form), as well as the results of the command execution and the * necessary information to determine how the command will be or was executed.