Use a virtualenv for remote operations by default

The docker python module fails to install without a virtualenv with
pip10.  The following error is seen in the task Ensure docker SDK for
python is installed:

Cannot uninstall 'ipaddress'. It is a distutils installed project and
thus we cannot accurately determine which files belong to it which would
lead to only a partial uninstall

This is fixed by using a remote Python virtual environment for executing
Kayobe Ansible tasks in the default Kayobe configuration.

Change-Id: I1348e428d736f93c76316c7fb5f2708b99554eb3
Story: 2002010
Task: 19648

doc/source/configuration/kayobe.rst

@@ -132,3 +132,7 @@ If kayobe detects that ``ansible_python_interpreter`` is set and references a
 virtual environment, it will create the virtual environment if it does not
 exist. Typically this variable should be set via a group variable for hosts in
 the ``seed``, ``seed-hypervisor``, and/or ``overcloud`` groups.
+
+The default Kayobe configuration in the ``kayobe-config`` repository sets
+``ansible_python_interpreter`` to ``{{ virtualenv_path }}/kayobe/bin/python``
+for the ``seed``, ``seed-hypervisor``, and ``overcloud`` groups.

etc/kayobe/inventory/group_vars/overcloud/ansible-python-interpreter

@@ -0,0 +1,3 @@
+---
+# Use a virtual environment for remote operations.
+ansible_python_interpreter: "{{ virtualenv_path }}/kayobe/bin/python"

etc/kayobe/inventory/group_vars/seed-hypervisor/ansible-python-interpreter

@@ -0,0 +1,3 @@
+---
+# Use a virtual environment for remote operations.
+ansible_python_interpreter: "{{ virtualenv_path }}/kayobe/bin/python"

etc/kayobe/inventory/group_vars/seed/ansible-python-interpreter

@@ -0,0 +1,3 @@
+---
+# Use a virtual environment for remote operations.
+ansible_python_interpreter: "{{ virtualenv_path }}/kayobe/bin/python"

releasenotes/notes/default-target-virtualenv-3a68472f66b7aaf3.yaml

@@ -0,0 +1,14 @@
+---
+fixes:
+  - |
+    Fixes an issue introduced by the release of ``pip`` version 10, whereby the
+    Python ``docker`` package would fail to install. This is resolved by using
+    a remote Python virtual environment for executing Kayobe Ansible tasks by
+    default. See `Story 2002010
+    <https://storyboard.openstack.org/#!/story/2002010>`__ for details.
+upgrade:
+  - |
+    Modifies the default ``kayobe-config`` configuration to use a remote Python
+    virtual environment for executing Kayobe Ansible tasks. This affects hosts
+    in the ``seed``, ``seed-hypervisor``, and ``overcloud`` groups.  See `Story
+    2002010 <https://storyboard.openstack.org/#!/story/2002010>`__ for details.