AssetsManager: improve performance of loading config groups

@Wikia/platform-team / @bognix / @kvas-damian 

Load groups config only on the first call to ``AssetsConfig::load()``.

```
Before the change:
  0.22% 0.003353     52 - AssetsConfig::loadGroups

After the change:
  0.03% 0.000277      1 - AssetsConfig::loadGroups
```