Set kernel logging to 'quiet' in AIOs

Currently the default CentOS value is to set printk to
7 4 1 7, showing up all the DEBUG messages.
OpenSUSE Leap 42.3 is using 1 4 1 7, while Ubuntu is
using 4 4 1 7.

This patch changes the default console_loglevel to 4 in AIOs
to only print KERN_WARNING, KERN_ERR, KERN_CRIT, and
KERN_ALERT, as this seems the best balance of reducing logging IO
while still exposing relevant errors in tests.

This patch also ensures users are aware that centos have a higher level
of kernel logging by default, so they can take their own decisions
about it.

Change-Id: I4ceb5b7d436b258656b0193dd5e2ef3efb44f592

deploy-guide/source/targethosts-prepare.rst

@@ -119,6 +119,13 @@ Configure CentOS
       # systemctl enable ntpd.service
       # systemctl start ntpd.service
 
+#. (Optional) Reduce the kernel log level by changing the printk
+   value in your sysctls:
+
+   .. code-block:: shell-session
+
+      # echo "kernel.printk='4 1 7 4'" >> /etc/sysctl.conf
+
 
 #. Reboot the host to activate the changes and use the new kernel.
 

tests/roles/bootstrap-host/templates/user_variables.aio.yml.j2

@@ -188,6 +188,12 @@ repo_build_pip_extra_indexes:
  - "{{ _pypi_wheel_mirror.stdout_lines[1] }}"
 {% endif %}
 
+# Set all the distros to the same value: a "quiet" print
+# of kernel log messages.
+openstack_user_kernel_options:
+  - key: 'kernel.printk'
+    value: '4 1 7 4'
+
 {% raw %}
 # We now have access to the branchname in testing variables.
 findgitreview: