Move exim role to top-level

Move the exim role to be a "generic" role in the top-level roles/
directory, making it available for use as a Zuul role.

Update the linters jobs to look for roles in the top level

Update the Role documentation to explain what the split in roles is
about.

Change-Id: I6b49d2a4b120141b3c99f5f1e28c410da12d9dc3

doc/source/roles.rst

@@ -5,4 +5,24 @@ Ansible Roles
 
 Documentation for roles included in `system-config`
 
+There are two types of roles.  Top-level roles, kept in the ``roles/``
+directory, are available to be used as roles in Zuul jobs.  This
+places some constraints on the roles, such as not being able to use
+plugins.  Add
+
+.. code-block:: yaml
+
+   roles:
+     - zuul: openstack-infra/system-config
+
+to your job definition to source these roles.
+
+Roles in ``playbooks/roles`` are designed to be run on the
+Infrastructure control-plane (i.e. from ``bridge.openstack.org``).
+These roles are not available to be shared with Zuul jobs.
+
+Role documentation
+------------------
+
+
 .. zuul:autoroles::

tox.ini

@@ -12,7 +12,7 @@ deps = -r{toxinidir}/test-requirements.txt
 [testenv:linters]
 whitelist_externals = bash
 commands =
-  bash -c "find playbooks/roles -maxdepth 1 -mindepth 1 -type d -print0 | \
+  bash -c "find roles/ playbooks/roles -maxdepth 1 -mindepth 1 -type d -print0 | \
      xargs -t -n1 -0 {toxinidir}/tools/check_readme.sh"
   flake8
   {toxinidir}/tools/run-bashate.sh
@@ -20,7 +20,7 @@ commands =
   python3 {toxinidir}/tools/irc_checks.py
   python3 {toxinidir}/tools/check_clouds_yaml.py
   # Ansible Lint Check
-  bash -c "find playbooks -type f -regex '.*.y[a]?ml' -print0 | xargs -t -n1 -0 \
+  bash -c "find roles playbooks -type f -regex '.*.y[a]?ml' -print0 | xargs -t -n1 -0 \
     ansible-lint -x ANSIBLE0004 -x ANSIBLE0006 -x ANSIBLE0007 -x ANSIBLE0011 \
                  -x ANSIBLE0012 -x ANSIBLE0013 -x ANSIBLE0015"