Add CLI for xOpera API

[anzoman]

Description

xOpera API is currently started with python3 -m opera.api.cli command which is not so practical.

We could create a CLI for xOpera API that would help users to get started easier:

The CLI commands could be the following:

# install the CLI
$ python3 -m venv .venv && . .venv/bin/activate
(.venv) $ pip install opera-api
# print OpenAPI specification
(.venv) $ opera-api openapi
# install prerequisites
(.venv) $ opera-api install
# run xOpera REST API
(.venv) $ opera-api run

Steps

TBD if this will be chosen for implementation.

Current behaviour

The opera-api is run from source code script.

Expected results

To be able to start the API with CLI command.