gate_hook: add support for dsvm-plus job type
gate_hook: add support for dsvm-plus job type

This hook will be utilized in gate to allow neutron project to decide
what is enabled in neutron '-plus' tempest job.

For now the only difference is qos service enabled, which uses the
neutron devstack plugin.

Change-Id: I9f73b938539586022aeb22e3a32877816e1dcaba

klmitch/neutron,bigswitch/neutron,openstack/neutron,huntxu/neutron,wolverineav/neutron,mahak/neutron,openstack/neutron,MaximNevrov/neutron,sebrandon1/neutron,noironetworks/neutron,mahak/neutron,sebrandon1/neutron,igor-toga/local-snat,huntxu/neutron,bigswitch/neutron,dims/neutron,MaximNevrov/neutron,noironetworks/neutron,wolverineav/neutron,openstack/neutron,eayunstack/neutron,dims/neutron,cloudbase/neutron,eayunstack/neutron,cloudbase/neutron,klmitch/neutron,igor-toga/local-snat,mahak/neutron