[cern] Strip signed certificate

cherry-picked from: https://review.openstack.org/#/c/574167/

Certificate (ca.crt) has to be striped for some application parsers as they might require pure base64 representation of certificate itself, without empty characters at the beginning nor the end of file

Change-Id: I5f58e19d03abdf040b9a5b5df2f4dd83b4c0e3a9 Closes-Bug: #1775342 (cherry picked from commit edee7030)

Closes OS-6907