Use "pip list" in check_libs_from_git

As described in the change, "pip freeze" has issues with the way
zuulv3 clones repos without a remote.  This is an attempt to use "pip
list" to check for local install

Change-Id: I33d25f86b6afcadb4b190a0f6c53311111c64521
diff --git a/stack.sh b/stack.sh
index f5aa765..c545c56 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1389,11 +1389,7 @@
 
 # ensure that all the libraries we think we installed from git,
 # actually were.
-#
-# NOTE(ianw) 2017-09-27 : "pip freeze" is currently having issues
-# with zuulv3 and the way it clones remotes.  We will restore this
-# with a slightly different check soon
-#check_libs_from_git
+check_libs_from_git
 
 
 # Configure nova cellsv2