use md5 auth by default

Summary

Md5 auth is used by default on user creation, on admin and guest creation and in connect CLI, and as a default auth when picodata expel executed

Besides, for guest user in bootstrap_entries we do Acl::ChangeAuth() to use Md5, this is required to ensure that in tarantool space guest is also authorized using Md5

Python Tarantool API does not provide a way to authenticate using Md5 (for now considered as a bug, should be resolved in future), that's why chap-sha1 auth method is set explicitly to allow running tests, this happens in tests when we use:

instance.create_user(...with_auth="chap-sha1")
instance.sql(CREATE USER...USING chap-sha1)

However in python tests when we connect using CLI it works perfectly and authorize using Md5:

    cli = pexpect.spawn(
        command=i1.binary_path,
        args=["connect", f"{i1.host}:{i1.port}", "-u", "testuser"],
        encoding="utf-8",
        timeout=1,
    )

• Close #876 (closed)
• Cherry-pick to: none
• Docs follow-up: docs#350 (closed)
• Depends on sbroad!512 (merged)