Fix haproxy agent unit test to be runnable alone by tox

haproxy agent uses periodic_interval option that can interfere
with the same option declared in neutron/service.py when running
'tox services.loadbalancer'

Change-Id: Ibcc91d603f5f31c7a376e0c735e8bf566198231d
Closes-Bug: #1221726
Merge "Fix haproxy agent unit test to be runnable alone by tox"