I guess this is because centos 5 is pretty old and probably does not have
let's encrypt root certificates installed.
Even surprised you were able to find a JDK 7 (or 8, better BTW) on this.
BTW for a master you probably want to use a more recent platform, and use
an agent if you need to build on
Actually resolved the issue. I used wget to download the key locally and
then was able to import. When using wget, had to use the flag to not check
certificate...
ERROR: cannot verify jenkins-ci.org's certificate, issued by `/C=US/O=Let's
Encrypt/CN=Let's Encrypt Authority X3':
Unable to
