* adding support for GPU backend to contrib/docker

added gpu dockerfiles

renamed Dockerfile for centos74

fixed NGRAPH_GPU_ENABLE cmake flag name

* Check for GPU support on the host system and fall back to CPU if not present

* removed double option for PREBUILT_LLVM

* updated README.md with additional references for GPU support

* added clarifying comments

cleaned up duplicate settings

* removed deprecated targets from the contrib/docker/Makefile

* resolved absolute vs. conditional assignment for variables based on reference OS

* removed example using a custom DOCKERFILE from README file