[code quality] Required environment variables are fetched using get()

[ilyash-b]

websocket-sessions-management/handlers/teleprinter/app.py

Lines 8 to 9 in 8557d26

	connections_table_name = os.environ.get('CONNECTIONS_TABLE_NAME')
	sessions_table_name = os.environ.get('SESSIONS_TABLE_NAME')

Should be os.environ['...']