Tag additional slow tests to run in slow gate.

This patch tags additional slow tests with @test.attr(type='slow')
so that they can run in the slow gate.

These tests are:
  - test_migration_live
  - test_cold_migration
  - test_volume_backup_create
  - test_volume_backup_get
  - test_volume_backup_restore
  - test_volume_backup_delete
  - test_detach_volume_to_instance

Change-Id: I45de98c301af560cd2bdae33daaaec50ed26132d
3 files changed