Clean up and enhance config for automated builds + tests

[tswilliams]

The config file for automated builds + tests - .gitlab-ci.yml - needs to be cleaned up (e.g. to remove duplication), and updated to:

• copy job output to EOS rather than AFS
• improve the directory structure for the jobs' output
• run build + test jobs for tags as well as for commits

TODO:

• Remove duplication across .gitlab-ci.yml, and simplify its contents where possible
• Move CI-specific files to ci subdirectory
• Write job output to EOS, and update 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

• Run build + test jobs for git tags

[tswilliams]

All done. Closing.