box: set box.cfg options via environment variables

Add ability to set box.cfg options via environment variables. These
variables should have name `TT_<OPTION>`. When Tarantool instance is
started under tarantoolctl utility, environment variables have higher
priority than tarantoolctl configuration file.

Closes #5602

Co-authored-by: Leonid Vasiliev <lvasiliev@tarantool.org>
Co-authored-by: Alexander Turenko <alexander.turenko@tarantool.org>

@TarantoolBot document
Title: Set box.cfg options via environment variables

Now, it is possible to set box.cfg options via environment variables.
The name of variable should correspond the following pattern:
`TT_<NAME>`, where `<NAME>` is uppercase box.cfg option name. For
example: `TT_LISTEN`, `TT_READAHEAD`.

Array values are separated by comma. Example:

```sh
export TT_REPLICATION=localhost:3301,localhost:3302
```

An empty variable is the same as unset one.