CI : Clean up config file, enable for tags, and update output directories

[tswilliams]

Addresses issue #51 ; specifically, this pull request:

• Removes duplication across .gitlab-ci.yml, and simplifies its contents
• Moves CI-specific files to ci subdirectory
• Move job output under EOS, and updates job output directory structure to ...

ci/
   (commits|tags)/
      <commit-id-or-tag-name>/
         latest     # symlink to most recently run pipeline for this commit/tag
         <pipeline-id>/
            api_uhal/
            yumrepos/
               slc6_x86_64/
               centos7_x86_64

• Enables build + test jobs for git tags