lua/log: add ability to configure logging early

In the patch we introduce an ability to configure logging
engine early without calling box.cfg{} at all. For this sake
we have to modify the load_cfg.lua (main cfg loader) together
with log.lua built in module.

In particular the configuration entries which are supposed to
be handled by modules are grouped in module_cfg table, where we
list box.cfg{} keys and methods to fetch/set the values. Same time
the template_cfg{} starts to carry 'module' keyword telling
prepare_cfg routine to use modules.

@TarantoolBot documnent
Title: Module log

To configure log module eary without initializing box module
at all call the `log.cfg({})` method, where the following
arguments are acceptable:

 - `log` to specify file, pipe or syslog (same as
   `box.cfg{log}` option); the default value is nil
   and stderr is used;

 - `level` to specify logging level; can be numbers (1-7) or
   strings (fatal,syserror,error,crit,warn,info,verbose,debug);
   the default value is 5 (i.e. 'info');

   While for symbolic representations of levels we allow only
   predefined keywords to be used, to keep backward compatibility
   the numbers can be any;

 - `format` to specify output encoding ('plain' or 'json');
   the default value is 'plain';

 - `nonblock` to open logging output in nonblocking mode
   (`true` or `false`); the default value is `false`.

Fixes #689

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>