Fix sphinx-docs job for stable branch

build-openstack-sphinx-docs jobs fail on stable branches due to the new
pip version 10 introduces some previously Warning as Error: in case of
calling "pip install" without any package name, the command fails.
tox_install.sh is called during docs job without any package passed to pip.

Change-Id: I391c779e4692d5de526dfda035843b8f5e267a75

tools/tox_install.sh

@@ -26,5 +26,10 @@ pip install -c"$localfile" openstack-requirements
 # install will be constrained and we need to unconstrain it.
 edit-constraints "$localfile" -- "$CLIENT_NAME"
 
+if [ -z "$@" ]; then
+    echo "No packages to be installed."
+    exit 0
+fi
+
 pip install -c"$localfile" -U "$@"
 exit $?