Add option in config to enable creation HA routers by default.
Also set max_l3_agents_per_router to 0, so HA router will be
scheduled on all L3 agents. This option is added unconditinally as
HA routers can be created without enabling l3_ha by default.
Change-Id: I88a2934cda6e958803dfee5334bdae265c0d7c6c