Revert "nova-multi-cell: Skip test_cold_migrate_unshelved_instance"

This reverts commit c11f4e5336.

Test should work since change:
    I0b81282eba8238d8b64a67e38cf9d6392de1f85c

Conflicts:
        devstack/nova-multi-cell-blacklist.txt

NOTE(aarents): the confict is due to change
I95bc22f7d65454cd9e7b54a0e6d9516f2f204978
Disable qos resize tempest test for nova-multi-cell job

Change-Id: Ib953470cf0ccddfec5f253097ddd7f7f0eaf4883

devstack/nova-multi-cell-blacklist.txt

@@ -5,9 +5,6 @@
 # only test advanced neutron features, unrelated to multi-cell testing.
 ^tempest.scenario.test_network
 
-# Skip until bug 1906428 is resolved
-tempest.scenario.test_shelve_instance.TestShelveInstance.test_cold_migrate_unshelved_instance
-
 # Also exlude resize and migrate tests with qos ports as qos is currently
 # not supported in cross cell resize case . See
 # https://bugs.launchpad.net/nova/+bug/1907511 for details