mount /etc/vomses from the host VM. Fix #8426

[cmsdmwmbot]

Jenkins results:

• Python3 Pylint check: succeeded
• Pycodestyle check: succeeded

Details at https://cmssdt.cern.ch/dmwm-jenkins/view/All/job/DMWM-CRABServer-PR-test/1998/artifact/artifacts/PullRequestReport.html

[belforte]

no need for much review. Already tested on crab-dev-tw01