tempest: Disable l3_agent_scheduler when running without L3 agent
tempest: Disable l3_agent_scheduler when running without L3 agent

Dragonflow can operate without L3 agent, and does not advertise L3 agent
scheduler extension when running this way

Change-Id: I23d0e558c8454636fcde0a1903c78965b70bc324

openstack-dev/devstack,dtroyer/devstack,dtroyer/devstack,nawawi/openstack,group-policy/devstack,openstack-dev/devstack,group-policy/devstack,noironetworks/devstack,noironetworks/devstack,nawawi/openstack