Update jenkins build slave packer script

Update jenkins build slave packer script

In this change,
 - Packer image build scripts updated with below entries in order to fix an issue
   with Let's Encrypt certificate when cloning repo from gerrit.est.tech due to DST
   Root CA X3 Expiration: https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/

   1. Remove outdated certificate from system:
      # rm -rf /usr/share/ca-certificates/mozilla/DST_Root_CA_X3.crt
   2. Update CA certificates on the system
      # sudo update-ca-certificates --fresh --verbose

Change-Id: I7a0eebe0e3ea6b7624b89e090cac566bca47e401