enable stack sudo for ironic discovery gate job
enable stack sudo for ironic discovery gate job

discovery_exercise.sh performs two steps that require superuser privileges:
* iptables -L ironic-inspector to confirm a node is not blacklisted before
  booting it for the discovery process
* virsh start <baremetal_domain> to trigger the discovery process of a node

This patch changes safe-devstack-vm-gate-wrap.sh behavior to avoid dropping
stack user from sudoers.d Similar approach has already been applied in case
of the nodepool.yaml set of jobs[1]

Links:
[1] https://github.com/openstack-infra/project-config/blob/master/jenkins/jobs/nodepool.yaml#L77

Change-Id: I69fb19f4baa0ae433ac53a4384992ee6a9d3ba65

Tesora/tesora-project-config,Tesora/tesora-project-config,openstack-infra/project-config,openstack-infra/project-config,dongwenjuan/project-config,dongwenjuan/project-config