* added Dockerfile.ngraph_cpp.centos74_cmake3, run_as_centos_user.sh, and run_as_ubuntu_user.sh to enable build/test with centos74

* added RUN_AS_USER variable as input for contrib/docker/Makefile

* moved dockerfile for centos7 to contrib/docker

* updated Makefile to take OS as a parameter from the command line for contrib/docker make targets

* fixed capability for setting OS variable on the command line for the contrib/docker/Makefile

* setting the NGRAPH_USE_PREBUILT_LLVM only if the gcc version is not 4.8

* setting NGRAPH_USE_PREBUILT_LLVM for all ubuntu builds

* updated contrib/docker/Makefile to enable the OS=centos74 override for make check_gcc