Storage: access token field is unused in system test 'test_create_signed_read_url_*_w_access_token'

[HemangChothani]

Currently in the system test blob.generate_signed_url method passing access_token=None, Due to that test_create_signed_read_url_*_w_access_token test not executed properly,so needs to pass value of access_token. also need to change the scope which passing to generate the access token as the current one is not working.