Add stages

Split CI jobs into separate installation stage and unittest and other things

• can pass coverage to subsequent tests for better coverage calculation
• allow failure for SE and CVMFS tests as they need a proxy
• fix bug in setest:cc7 test which was missing dirac-proxy-init -x before dirac-configure
• README: fix badges for gitlab update
• backport pip install from master
• prepare for automatic creation of docs and tags in CI