Added CentOS7 test configuration to .gitlab-ci.yml and removed Slc6. Maybe fixed missing pyxrootd installation...

merge fixed unit tests into master branch

See merge request !1

Changed known_hosts.yml import function to raise Exceptions instead of returning -1 and adjusted unit tests

# Conflicts:
#   .gitlab-ci.yml

* added some modifications to save input files to classAd of condor -> grid contorl is sometimes to fast with clean up

* added InputFiles_tmp to classAds -> GridControl was to fast deleting input list (needed for monitoring)