BE-1 : Check if the username is already stored in our system

[omonimus1]

Problem
Actually, the application asks always for a username even if it is already stored in the user_data.json

**Solution **
Check if the .json file that contains the username exists. If the file exists, fetch the username value, otherwise, ask the username and store it in user_data.json file to create data persistency.

[omonimus1]

If the user name is already stored, say hello to the user.
Otherwise, ask to user his/her name and story it.