* remove custom install path

* fix travis build

* Add NGRAPH_INSTALL_PREFIX as an alias for CMAKE_INSTALL_PREFIX to make our unit tests pass.

* change install path setting

* 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

* saved a simplified contrib/docker/Makefile and helper scripts

* fixed flow for make targets

* restored original definition for setting the PARALLEL option on the command line per github comments

* remove double build for make install targets

* added a save for the ngraph_dist_gcc.tgz to maintain existing behavior

* fixed passing the PARALLEL value to the make targets

* integrated latest working build-ngraph-and-test script

* integrated the latest working Makefile

* removed reference to the THIRD_PARTY_CACHE_DIR (for future)

* updated the contrib/docker/README.md file