Create a Tool.default_command method #1077

To avoid duplication between (and within) get_bitbucket_steps methods for CI, and probably help messages, etc.

This would be backend dependent.

By default, it should return None, since not all tools have commands. Otherwise, return a string.

The individual tool implementations (Tool subclassses), for tools which associated with commands, should provide an implementation.

We should use the method anywhere where the default command is used, e.g. help messages, but certainly get_bitbucket_steps methods.