Stop running stable/folsom jobs

The OpenStack Havana release heralds end of support for Folsom, so
stop testing it.

* modules/jenkins/files/slave_scripts/docbook-properties.sh: Remove
folsom-specific settings.

* .../openstack_project/files/gerrit/acls/openstack/ceilometer.config:
Get rid of the stable/folsom ACL override.

* .../jenkins_job_builder/config/devstack-gate.yaml
* .../manuals.yaml
* .../projects.yaml
* .../python-bitrot-jobs.yaml
* modules/openstack_project/files/zuul/layout-dev.yaml
* .../layout.yaml: Remove all folsom-only jobs and clean up
configuration and comments associated with them.

* tools/invite2summit/settings.py.sample: Adjust usage comment to no
longer refer to Folsom.

Change-Id: Ieb38a74e6945abe2e40cd3e1cd23b52fbd826933
Merge "Stop running stable/folsom jobs"