Move live_migration test hooks under gate/

This patch resolves a TODO in the .zuul.yaml about using common
irrelevant files in our dsvm jobs. To be able to do that we need to move
the test hooks from nova/tests/live_migraton under gate/.

Change-Id: I4e5352fd1a99ff2b4134a734eac6626be772caf1

playbooks/legacy/nova-grenade-live-migration/run.yaml

@@ -48,7 +48,7 @@
               export OVERRIDE_ZUUL_BRANCH=$BRANCH_OVERRIDE
           fi
           function post_test_hook {
-              /opt/stack/new/nova/nova/tests/live_migration/hooks/run_tests.sh
+              /opt/stack/new/nova/gate/live_migration/hooks/run_tests.sh
           }
           export -f post_test_hook
           cp devstack-gate/devstack-vm-gate-wrap.sh ./safe-devstack-vm-gate-wrap.sh

playbooks/legacy/nova-live-migration/run.yaml

@@ -48,7 +48,7 @@
           export DEVSTACK_GATE_TEMPEST_NOTESTS=1
           export DEVSTACK_GATE_TOPOLOGY="multinode"
           function post_test_hook {
-              /opt/stack/new/nova/nova/tests/live_migration/hooks/run_tests.sh
+              /opt/stack/new/nova/gate/live_migration/hooks/run_tests.sh
               $BASE/new/nova/gate/test_evacuate.sh
           }
           export -f post_test_hook