Add a note about relogging in as the stack user when docker commands fail

We currently do the following to add the 'stack' user on the undercloud
to the 'docker' group so it can access the docker daemon (when the local
docker registry option is enabled):
...
if str2bool(hiera('enable_docker_registry', true)) {
  group { 'docker':
    ensure => 'present',
  }
  user { 'docker_user':
    name => hiera('tripleo_install_user'),
    groups => 'docker',
    notify => Service['docker'],
  }
  include ::tripleo::profile::base::docker_registry
}
...

For this to work the stack user needs to logoff and logon again.
Closes-Bug: #1722498

Change-Id: Ia1f76f61828c119667879c41f1ad5d86b8901adb

doc/source/install/installation/installing.rst

@@ -129,3 +129,9 @@ so it is not necessary to copy them to ``undercloud.conf``.
     ``openstack undercloud install`` can be rerun to reapply changes from
     undercloud.conf to the undercloud. Note that this should **not** be done if an
     overcloud has already been deployed or is in progress.
+
+.. note::
+   If running ``docker`` commands as a stack user after an undercloud install fail
+   with a permission error, log out and log in again. The stack user does get added
+   to the docker group during install, but that change gets reflected only after a
+   new login.