zsh openrc compatibility

Replacing $BASH_SOURCE with ${BASH_SOURCE:-$0} makes devstack zsh friendly: in
bash, $BASH_SOURCE is used per usual; in zsh, where $BASH_SOURCE isn't
defined, $0 is used, which, unlike in bash, evaluates to the current
source file.

Now you can source devstack's openrc from a zsh shell.

Tested with bash and zsh from directories other than the root devstack
directory.

Change-Id: Iab1a817b15d86144163b5094bb58f94b15c598a0
diff --git a/stackrc b/stackrc
index 0151672..4da481e 100644
--- a/stackrc
+++ b/stackrc
@@ -1,7 +1,7 @@
 # stackrc
 #
 # Find the other rc files
-RC_DIR=$(cd $(dirname "$BASH_SOURCE") && pwd)
+RC_DIR=$(cd $(dirname "${BASH_SOURCE:-$0}") && pwd)
 
 # Destination path for installation
 DEST=/opt/stack