job cache now varies by remote url

When updating different Jenkins installations, JJB would end up
invalidating the job cache.  By adding the jenkins.url in the cache file
name, we can maintain a job cache per remote Jenkins instance.

Name of the cache file is based on Jenkins URL.

Change-Id: Ib9c08d8029267729e7a030a4e8bc0a9f05eb8ce3
Reviewed-on: https://review.openstack.org/24404
Reviewed-by: Arnaud Fabre <fabre.arnaud@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins