Convert legacy nova-live-migration and nova-multinode-grenade to py3

This makes these legacy devstack-gate-based jobs run with python3.

Change-Id: Id565a20ba3ebe2ea1a72b879bd2762ba3e655658

gate/live_migration/hooks/ceph.sh

@@ -6,8 +6,9 @@ function prepare_ceph {
     source /tmp/devstack-plugin-ceph/devstack/lib/ceph
     install_ceph
     configure_ceph
-    #install ceph-common package on compute nodes
+    #install ceph-common package and additional python3 ceph libraries on compute nodes
     $ANSIBLE subnodes --become -f 5 -i "$WORKSPACE/inventory" -m raw -a "executable=/bin/bash
+    USE_PYTHON3=${USE_PYTHON3:-True}
     source $BASE/new/devstack/functions
     source $BASE/new/devstack/functions-common
     git clone https://opendev.org/openstack/devstack-plugin-ceph /tmp/devstack-plugin-ceph

gate/live_migration/hooks/run_tests.sh

@@ -56,6 +56,7 @@ echo '4. test with Ceph for root + ephemeral disks'
 # Discover and set variables for the OS version so the devstack-plugin-ceph
 # scripts can find the correct repository to install the ceph packages.
 GetOSVersion
+USE_PYTHON3=${USE_PYTHON3:-True}
 prepare_ceph
 GLANCE_API_CONF=${GLANCE_API_CONF:-/etc/glance/glance-api.conf}
 configure_and_start_glance