Default CIRROS_ARCH to host arch
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

nawawi/openstack,openstack-dev/devstack,openstack-dev/devstack,nawawi/openstack