[Rally] Load Neutron-LBaaS extension as devstack plugin
[Rally] Load Neutron-LBaaS extension as devstack plugin

Devstack has removed LBaaS extension loaded as service,
so we have to load it as devstack plugin.

Change-Id: Ie7a2509f2393abf83f3c1197ecd4fd2461b21b39

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