config: `include` section for config files.

Using this section it is possible to include one or several other config
files from any config file. This is a top level field for file-based
configuration. If present, this field must be an array of strings.
Each entry in this array can be an absolute or relative path to a config
file to be included, or a wildcard pattern. Relative path are considered
relative to the file, where they are included.
Include section can be placed inside a conditional section.
The non-existing included files are skipped. If an included file exists
and is not a valid tarantool config file, an error will be raised.

Example config.yaml:

include:
  - config_a.yaml
  - /etc/tarantool/config_b.yaml
  - conf.d/*.yaml

For the provided example the resulting configuration will be merged
according to the rules:

1. Config file provided by --config flag is read (config.yaml in
   our example).
2. config_a.yaml from the same directory as the config.yaml is
   merged onto it.
3. Recursively, the includes of config_a.yaml are merged onto the
   configuration.
4. /etc/tarantool/config_b.yaml is merged.
5. Recursively, the includes of /etc/tarantool/config_b.yaml are
   merged onto the configuration.
6. For each config file, matching the wildcard pattern, in alphabetical
   order the configuration from the file itself is merged, then its
   includes are processed recursively, before processing the next
   wildcard match.
   Requested by @grafin in https://github.com/tarantool/tarantool/commit/e204545fe4ea0d764298760aa83a5a7968f1c5c9.