Resolve "Check environment variables at startup"

Description

Closes #94

Changelog

• Add env check at init
• Add DBSalt class for managing DB_SALT variable
• Update unit tests with dummy environment variables
• Add SSHKeyfile class for managing ssh key file
• Check required files (email, scaleway configs, ssh key file)

How to test

For env

• Launch backend with all variables set in shell => test it is ok
• Launch backend with all variables set in backend.env file => test it is ok
• Launch backend with a missing variable => test that it exits with error message

For files

• give an incorrect VM_ID_FILE value => test that it exits with error message
• remove email and scaleway json files in configs => test that it exits with error message