rename system Config to system Trait.

command used:
```
find frame/ bin/ test-utils/ utils/ -name *.rs -exec sed -i 's/system::Trait>::/system::Config>::/g' {} \;
find frame/ bin/ test-utils/ utils/ -name *.rs -exec sed -i 's/impl frame_system::Trait for /impl frame_system::Config for /g' {} \;
find frame/ bin/ test-utils/ utils/ -name *.rs -exec sed -i 's/impl system::Trait for /impl system::Config for /g' {} \;
```
plus some manual ones especially for frame-support tests and frame-system