Added admin menu configuration in yml file

Automatically menu building could looks like good idea but it have few disadvantages like for example there is no way to manage elements orders. 
From this PR menu will be build from .yml file configuration:

Example configuration: 
```
# app/config/admin_menu.yml
menu:
  - news
  - news_categories
  - admin.menu.structure:
    - home_page
    - about_us_page
  - galleries
``` 

Configuration by default is read from ``"%kernel.root_dir%/config/admin_menu.yml"`` path but it could be changed like in following example 

```
fsi_admin:
  menu_config_path: %kernel.root_dir%/config/custom_menu.yml
``` 

This PR should simplify a bit manager and menu builder code.