Switch magnum to release-openstack-server
Switch magnum to release-openstack-server

Following up [1], do not release magnum to pypi
and set to release-openstack-server.

[1] I50b1cfb3d892443cdb3faffe2f931914b0b47684

Change-Id: Ibc5a86d754266454e5d560307b6a169d156dd1c1

openstack-infra/project-config,openstack-infra/project-config