Add unit tests for the service

[LeonidVas]

Now the tests is absent and such code is difficult to develop and maintain.
To test interaction with S3 the "tarantool-test" bucket can be used.

[Totktonada]

How about using minio?

[LeonidVas]

I have spent some time thinking about this task, and it's logical to divide this tasks into two:

• unit tests (can also be divided)
• integration tests

I propose leaving the unit test part of the work here, and move the integration test part of the work into a separate task.

In the context of this task, I propose to add the following tests:

• check if the configuration is loaded correctly (including gpg key)
• implement testing controller and view according to https://flask.palletsprojects.com/en/2.0.x/testing/ . For this, the model must be mocked
• add testing of model by using minio without checking of the synchronization (this part will be checked by integration tests)

Probably need to change some of the application code while tests will writes. These changes should be as minimal as possible, since there is no guarantee that the correctness of the code will be maintained in the form of tests at this time.