tt 2.0 configuration format

[p7nov]

Product: tt cli
Since: tt 2.0
Audience/target: all
Root document: https://www.tarantool.io/en/doc/latest/reference/tooling/tt_cli/configuration
SME: @ psergee, @ YanaPod

Details

From tt 2.0.0. changelog:

• Global flags are required to be positioned only before child
  commands. Example: tt --cfg tt.yaml install tt.
• tt config format: separate tt environment options from application options.

Revise the tt config structure description to reflect the actual structure of tt 2.0+

Definition of done

• The tt config description reflects the actual structure of tt 2.0+ config
• the Global flags page tells how to pass these flags.