config: change default paths to var/<...>

The new default directory layout is the following.

```
+ var/
  + lib/
    + instance-001/
      - *.xlog
      - *.snap
      - *.vylog
  + log/
    + instance-001/
      - tarantool.log
  + run/
    + instance-001/
      - tarantool.control
      - tarantool.pid
```

Our guess is that it should be convenient for development environments,
when the application consists of several instances. The idea is borrowed
from the `cartridge-cli` and `tt` tools.

We plan to synchronize these defaults with the `tt` tool, to simplify
cases, when pure tarantool (without `tt`) should be run in the
directories layout created by `tt`. It should simplify debugging using
`gdb`, `strace` and other tools.

Also, it should reduce possible confusion for users of `cartridge-cli`
and `tt`.

Part of #8862

NO_DOC=https://github.com/tarantool/doc/issues/3544 already points to
       the actual instance config schema