Add global venv enable/disable knob

Adds USE_VENV to globally enable/disable use of virtual environments.

ADDITIONAL_VENV_PACKAGES is used to manually add packages that do not
appear in requirements.txt or test-requirements.txt to be installed
into each venv.  Database Python bindings are handled this way when
a dataabse service is enabled.

Change-Id: I9cf298b936fd10c95e2ce5f51aab0d49d4b7f37f
diff --git a/doc/source/configuration.rst b/doc/source/configuration.rst
index 7d06658..05a21cd 100644
--- a/doc/source/configuration.rst
+++ b/doc/source/configuration.rst
@@ -170,6 +170,30 @@
 
       LIBS_FROM_GIT=python-keystoneclient,oslo.config
 
+Virtual Environments
+--------------------
+
+  | *Default: ``USE_VENV=False``*
+  |   Enable the use of Python virtual environments by setting ``USE_VENV``
+      to ``True``.  This will enable the creation of venvs for each project
+      that is defined in the ``PROJECT_VENV`` array.
+
+  | *Default: ``PROJECT_VENV['<project>']='<project-dir>.venv'*
+  |   Each entry in the ``PROJECT_VENV`` array contains the directory name
+      of a venv to be used for the project.  The array index is the project
+      name.  Multiple projects can use the same venv if desired.
+
+  ::
+
+    PROJECT_VENV["glance"]=${GLANCE_DIR}.venv
+
+  | *Default: ``ADDITIONAL_VENV_PACKAGES=""``*
+  |   A comma-separated list of additional packages to be installed into each
+      venv.  Often projects will not have certain packages listed in its
+      ``requirements.txt`` file because they are 'optional' requirements,
+      i.e. only needed for certain configurations.  By default, the enabled
+      databases will have their Python bindings added when they are enabled.
+
 Enable Logging
 --------------
 
diff --git a/doc/source/index.rst b/doc/source/index.rst
index bac593d..d98e573 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -210,6 +210,8 @@
 -----
 
 * `tools/build\_docs.sh <tools/build_docs.sh.html>`__
+* `tools/build\_venv.sh <tools/build_venv.sh.html>`__
+* `tools/build\_wheels.sh <tools/build_wheels.sh.html>`__
 * `tools/create-stack-user.sh <tools/create-stack-user.sh.html>`__
 * `tools/create\_userrc.sh <tools/create_userrc.sh.html>`__
 * `tools/fixup\_stuff.sh <tools/fixup_stuff.sh.html>`__