Allow override of python-brick-cinderclient-ext library used by cinder

Added to requirements:
https://review.openstack.org/309084

Functional tests were added
https://review.openstack.org/265811

But they still use the version of python-brick-cinderclient-ext from pip.

This change updates devstack to pull in the changes from
python-brick-cinderclient-ext patch sets instead, when configured to do so.

Change-Id: I6d0f09950ea1200d3367a53aa4a3eea9be7abc66
Needed-by: I34f3b5ceaad7a50b1e9cadcc764f61c0aabe086d
diff --git a/lib/cinder b/lib/cinder
index 9c818c6..de67593 100644
--- a/lib/cinder
+++ b/lib/cinder
@@ -40,6 +40,7 @@
 # set up default directories
 GITDIR["python-cinderclient"]=$DEST/python-cinderclient
 GITDIR["os-brick"]=$DEST/os-brick
+GITDIR["python-brick-cinderclient-ext"]=$DEST/python-brick-cinderclient-ext
 CINDER_DIR=$DEST/cinder
 
 # Cinder virtual environment
@@ -466,6 +467,11 @@
 
 # install_cinderclient() - Collect source and prepare
 function install_cinderclient {
+    if use_library_from_git "python-brick-cinderclient-ext"; then
+        git_clone_by_name "python-brick-cinderclient-ext"
+        setup_dev_lib "python-brick-cinderclient-ext"
+    fi
+
     if use_library_from_git "python-cinderclient"; then
         git_clone_by_name "python-cinderclient"
         setup_dev_lib "python-cinderclient"