Clarify names of three layers

- Correct names of layers that have unclear names for historical reasons:
  - Rename #roads to #roads-low-zoom
  - Rename #minor-roads-casing to #roads-casing
  - Rename #minor-roads-fill to #roads-fill 

This pull request solves the following problem:
- The layers #roads and #minor_roads are confusingly named. 

This pull request changes the rendering as follows:
- No change.