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config: handle box.cfg's TT_* env vars

There may be some confusion, so let's start with a background
information.

There are `TT_*` environment variables introduced in commit 1b330121ec7d
("box: set box.cfg options via environment variables"). They're
interpreted by the `box.cfg()` call.

There are `TT_*` environment variables introduced in commit 82b0cff9d63b
("config: introduce env source"). They're interpreted by the declarative
configuration logic, when tarantool starts with the `--name <...>` CLI
option.

box.cfg's env variables have names deduced from box.cfg option names,
while config's env variable names are deduced from the config schema.

Some options have the same names here and there, for example
`TT_REPLICATION_ANON` (from `box.cfg.replication_anon` and
`replication.anon`). However, there are ones that have different names,
for example `TT_LISTEN` and `TT_IPROTO_LISTEN`.

Moreover, the declarative configuration has its own restrictions on the
configuration data. For example, `TT_IPROTO_LISTEN` is always a list of
URIs (like `[{"uri": <...>, "params": {<...>}}]`), not a single URI, not
a string, not a number. The declarative configuration has a certain
shape and doesn't allow polymorphic values.

Next, handling of box.cfg's variables by the old code in `load_cfg.lua`
doesn't work well with the declarative configuration flow.

The main reason is that the new configuration flow calls `box.cfg()`
with all the `box.cfg` values set, including default ones. If a user
removes an option from its config, it applies its default. On the same
time it instructs `box.cfg()` to don't read the corresponding
environment variables.

This commit offers a partial solution: it adds support of the most of
the box.cfg environment variables. The values are added into the
configuration data with the lowest priority: if the same value is set
in, for example, a file configuration, the file's value is preferred.

The following box.cfg's environment variables are not... (continued)

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