[Docker] more base images (#10560)
		
			
			* Revert "work around an issue with docker/dockerhub (#10547)"
This reverts commit 0ef139ac4d.
* Add more docker base image configurations.
 - Renamed configs
   - "centos" -> "centos-7"
   - "fedora" -> "fedora-24"
   - "opensuse" -> "opensuse-tumbleweed"
   - "scilinux" -> "scilinux-7"
   - "ubuntu" -> "ubuntu-1804"
 - Added configs
   - "centos-6"
   - "fedora-29"
   - "opensuse-leap"
   - "scilinux-6"
   - "ubuntu-1604"
With these changes, our docker hub image
collection will consist of the following:
 - "spack/archlinux"
   - "latest"
 - "spack/centos"
   - "latest", "centos7", "7"
   - "centos6", "6"
 - "spack/fedora"
   - "latest", "29"
   - "24"
 - "spack/opensuse"
   - "latest", "tumbleweed"
   - "leap"
 - "spack/scilinux"
   - "latest", "7"
   - "6"
 - "spack/ubuntu"
   - "latest", "bionic", "18.04"
   - "xenial", "16.04"