Handle multi-cell deployments in live-migration scenario tests

In multi-cell Nova deployments, live migration between cells is not supported. Several scenario tests (zone_migration, host_maintenance) select a destination compute host for live migration that could belong to a different cell, causing intermittent failures in multi-cell environments.

These tests now rely on the tempest options migration_source_host, migration_dest_host and can_migrate_between_any_hosts to let operators pin the source and destination hosts (a controlled, same-cell pair) or declare that any host pair is safe.

A shared helper, skip_if_missing_migration_hosts(), is added to the scenario base class so it can be reused by any test. Directed-migration tests are skipped unless the configuration is consistent:

• both migration_source_host and migration_dest_host are set, or
• neither is set together with can_migrate_between_any_hosts enabled.

Setting only one host is skipped because Nova could place the instance on the destination host (making source and destination the same), and leaving both unset without can_migrate_between_any_hosts could select hosts in different cells.

New “without destination” test variants are added that do not pin a destination host, letting the zone_migration strategy select it, which is safer in multi-cell deployments:

• test_execute_zone_migration_volume_and_compute_without_destination
• test_execute_zone_migration_volume_and_compute_bfv_without_destination

Instances that do not require volume attachment are now created with _create_instance (admin client, waiting for ACTIVE), consistent with the other scenario tests, instead of create_server.