T-API: OpenCL device selection via environment variable

Layout: `<Platform>:<CPU|GPU|ACCELERATOR|nothing=GPU/CPU>:<deviceName>`

Samples:
```
- OPENCV_OPENCL_DEVICE=AMD:GPU:Tahiti
- OPENCV_OPENCL_DEVICE=:GPU:1
- OPENCV_OPENCL_DEVICE=:CPU:
- OPENCV_OPENCL_DEVICE=AMD
- OPENCV_OPENCL_DEVICE=:GPU|CPU
```