box: add box.cfg.auth_type configuration option

This commit adds a new box configuration option - box.cfg.auth_type.
The option takes an authentication method name (string) that will be
used by box.schema.user.passwd to generate user authentication data.

The only authentication method supported by Community Edition (CE)
is 'chap-sha1' so we don't document or announce this feature in CE.
More methods and tests will be added to Enterprise Edition (EE).

Part of #7988

NO_DOC=ee
NO_CHANGELOG=ee