Add the "env" option to config:get()

Since: 3.7.0

The new option allows controlling whether TT_* environment variables
are taken into account when retrieving configuration values.

Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/config/#lua-function.config.get

• The env option controls the usage of TT_* environment variables
  in config:get().
• By default, environment variables are taken into account when the
  instance option is not specified.
• If env is set to false, environment variables are ignored even
  when the instance option is not specified.
• If env is set to true together with the instance option, an
  error is raised.
  Requested by @mandesero in https://github.com/tarantool/tarantool/commit/5f1f5af08f8c29147515bea54f878be9feffef4f.

Start with the config:get() section at the linked root document and locate the documentation source for that page. Document the env option, its default behavior, how false disables TT_* variables, and that true with instance raises an error; done when the page reflects all listed rules.