Add option to download all libs from git

For client debugging that invokes multiple libs it can be useful
to have all libs directly in git and not listing all of them
in LIBS_FROM_GIT.

TrivialFix

Change-Id: Ie631cc4045231ebbe8177d2d113e47e4bf83f61c
diff --git a/doc/source/configuration.rst b/doc/source/configuration.rst
index 1f5797c..384ceee 100644
--- a/doc/source/configuration.rst
+++ b/doc/source/configuration.rst
@@ -427,6 +427,9 @@
 
       LIBS_FROM_GIT=python-keystoneclient,oslo.config
 
+Setting the variable to ``ALL`` will activate the download for all
+libraries.
+
 Virtual Environments
 --------------------