v1.3.0-rc.1

• Update build system to accept a TAG variable for things like rc.x
• Add common CI tests for things like golint, gofmt, unit tests, etc.
• Revert "Add ability to merge envars of the form NVIDIA_VISIBLE_DEVICES_*"
• Flip build-all targets to run automatically on merge requests
• Rename github.com/NVIDIA/container-toolkit to nvidia-container-toolkit
• Add new config options to pull device list from mounted files instead of ENVVAR