Refactor ITDE startup functions

[ckunki]

See also

• Reconnect/Disconnect the Ai-Lab container from the Docker-DB during start/stop Docker-DB #89
• connecting new ai-lab container to existing docker-db network #92

Acceptance criteria

In file exasol/nb_connector/itde_manager.py

• Rename start_itde() to restart_itde()?
• Rename method is_itde_running() to is_itde_ready().
• Refactor function is_itde_running() to return an enum rather than a tuple (see below)

class ItdeContainerStatus(Enum):
    EXISTS = 0
    RUNNING = 1

This would also enable to handle the case not container_name or not network_name more explicitly by returning None or having an explicit enum value UNKNOWN or INVALID.