Add back a explicit barbican-worker.py install line.

If this line is not included in the spec file, then a
'Requires: /var/lib/jenkins/.pyenv/shims/python' line shows up in the
RPM builder output, and then the package doesn't install correctly.
This commit is to add this line back into the spec file.

Change-Id: I5e7cf15525c5681bef3034a06339982b373578d6