Pass in GIT_BASE when calling stack.sh

since ./stack.sh is called as a binary, $GIT_BASE needs to be passed in
to override devstack's default GIT_BASE.

And add comment about git.openstack.org

Change-Id: I55482a387fe54235baba90e7b83837f7cb5b4bc6

grenade.sh

@@ -172,7 +172,7 @@ if [[ "$RUN_BASE" == "True" ]]; then
 
     echo_summary "Running base stack.sh"
     cd $BASE_DEVSTACK_DIR
-    ./stack.sh
+    GIT_BASE=$GIT_BASE ./stack.sh
     stop $STOP stack.sh 10
 
     # Cache downloaded instances

grenaderc

@@ -5,6 +5,7 @@ RC_DIR=$(cd $(dirname "$BASH_SOURCE") && pwd)
 
 # Base GIT Repo URL
 # Another option is http://review.openstack.org/p
+# Another option is https://git.openstack.org
 GIT_BASE=https://github.com
 #GIT_BASE=/opt/git