Add CLI to the Repository Management Tools

[vladimir-v-diaz]

The repository management tools may either be imported into a Python module or used with the Python interpreter in interactive mode.

Build a CLI on top of the available functions/methods for users that prefer to also use the command-line. Consider the use cases for the CLI: for simpler repositories? Support all of the capabilities of the current tools? The Go implementation of TUF provides a good example of how the CLI should behave and what to support.

[lukpueh]

Closing due to uncertain future of TUF cli. Re-visit with #811.