Default CIRROS_ARCH to host arch

This change use uname -m to get the portable host arch and uses that
as a new default. on x86_64 hosts this should result in no visable change
in behavior however on a non x86 host it will cause devstack to attempt
to download a cirros image that matches the host.

Change-Id: I6d1495a23400ef4cf496302028324fa5794dd45f
diff --git a/stackrc b/stackrc
index 681e9de..e48fd81 100644
--- a/stackrc
+++ b/stackrc
@@ -663,7 +663,7 @@
 #IMAGE_URLS="http://download.cirros-cloud.net/${CIRROS_VERSION}/cirros-${CIRROS_VERSION}-${CIRROS_ARCH}-disk.img" # cirros full disk image
 
 CIRROS_VERSION=${CIRROS_VERSION:-"0.5.2"}
-CIRROS_ARCH=${CIRROS_ARCH:-"x86_64"}
+CIRROS_ARCH=${CIRROS_ARCH:-$(uname -m)}
 
 # Set default image based on ``VIRT_DRIVER`` and ``LIBVIRT_TYPE``, either of
 # which may be set in ``local.conf``.  Also allow ``DEFAULT_IMAGE_NAME`` and