For example we can currently define a global variables as MY_VAR=$WORSPACE/foobar and when we will run the Jenkins job it will contain the correct path.

Still, if we try to expand the HUDSON_COOKIE variable nothing will happen and we would end-up without any expansion being made

This is a real issue because it prevents us from implementing a workaround for avoiding SSH session highjacking between multiple builds.

In order to avoid highjacking we need to be sure that two builds will not share a SSH session by accident. At this moment the only safe way to distinguish between two builds is the HUDSON_COOKIE variable. Other variables are not reliable because they could be too long,
ANSIBLE_SSH_CONTROL_PATH='%(directory)s/%r@%h:%p.$HUDSON_COOKIE-/home/rhos-ci/jenkins/workspace/foobar'
ANSIBLE_SSH_CONTROL_PATH_DIR='~/.ssh/'
The SSH control path is a file socket and its full path is limited to 108 characters, which means that if the expansion would work this could address the issue.