Live migration invoked from Horizon skips scheduler while using metadata

Originally reported for Ocata, I believe this is still a problem.

Let's assume cirros image and 2 compute nodes (osp10-comp01, osp10-comp02).

1. upload cirros image.

2. set os_distro=cirros to the image.
[stack@osp10-director work(overcloud_admin):v2]$ openstack image set --os-distro cirros cirros

3. Create 2 Host Aggregates named 'ag_cirros' and 'ag_fedora', and set 'os_distro=cirros', 'os_distro=fedora' metadata respectively.
[stack@osp10-director work(overcloud_admin):v2]$ openstack aggregate create ag_cirros --property os_distro=cirros
[stack@osp10-director work(overcloud_admin):v2]$ openstack aggregate create ag_fedora --property os_distro=fedora

4. Set osp10-comp02 to ag_cirros Host Aggregate and osp10-comp01 to ag_fedora Host Aggregate.
[stack@osp10-director work(overcloud_admin):v2]$ openstack aggregate add host ag_cirros osp10-comp02.osptest.local
[stack@osp10-director work(overcloud_admin):v2]$ openstack aggregate add host ag_fedora osp10-comp01.osptest.local

5. Create an instance named 'vm1' from cirros image. The instance is scheduled to osp10-comp02 by AggregateImagePropertiesIsolation.

6. Run 'nova live migration' CLI command for the instance to migrate to osp10-comp01 who is in a Host Aggregate with 'os_distro=fedora' metadata.
The migration fails as expected because the Host Aggregate osp10-comp01 is in doesn't have 'os_distro=cirros' metadata.

7. Do live migration from Horizon for the same instance to migrate to osp10-comp01 who is in a Host Aggregate with 'os_distro=fedora' metadata. (specifying "New Host" form to 'osp10-comp01')
The migration suceeded to migrate to osp10-comp01. This is not an unexpedted result.
It seemed that the instance didn't scheduled by nova-scheduler from logs.

Actual results:
Migration is getting successful from Horizon.

Expected results:
Migration should not be completed from Horizon as well.