Fix error on devstack cleanup

Change I7ebf4137feb04827490dffc0dac3d6e4c8888075 added 'set -e' in
devstack/plugin.sh, but on devstack cleanup, some commands may fail
because of non-working services (i.e after a reboot).
This commit allows 'openstack keypair delete' to fail on clean up.

Change-Id: Ic782faba3eb907d29b6735ac0a6d6a8a2e104e00

devstack/plugin.sh

@@ -654,7 +654,7 @@ function octavia_cleanup {
     fi
     if [ $OCTAVIA_NODE == 'main' ] || [ $OCTAVIA_NODE == 'standalone' ] ; then
         if [ ${OCTAVIA_AMP_SSH_KEY_NAME}x != x ] ; then
-            openstack keypair delete ${OCTAVIA_AMP_SSH_KEY_NAME}
+            openstack keypair delete ${OCTAVIA_AMP_SSH_KEY_NAME} || true
         fi
     fi
     if [[ "$OCTAVIA_USE_MOD_WSGI" == "True" ]]; then