Add new env to enable LXD on gate
Add new env to enable LXD on gate

Specify new env on LXD gate which will be used by plugin.sh/gate hook to enable
nova-lxd and create proper image to be used.

Change-Id: I37415752195dd7b82e6dc45a2360d849f7532b58

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