Mark fullstack test_l2_agent_restart as unstable
Mark fullstack test_l2_agent_restart as unstable

With linuxbridge agent the connectivity test fails often after l2 agent
restart, so mark it as unstable until the root cause is found.

Change-Id: Ida3193acac2fa22bbfd4b18ef3a3bceafe46d3ec
Related-Bug: #1928764

mahak/neutron,mahak/neutron,openstack/neutron,openstack/neutron,mahak/neutron,openstack/neutron