Fix diff return code in hosts element

diff exits non-zero when a diff actually exists. This causes 51-hosts to
exit and fails the os-refresh-config run. This issue is manifested when
doing a stack-update and a new host is added to the stack.

Closes-Bug: #1355646

Change-Id: Ia0e3670960b4b136fda9b09c188baf1f23d45d6b