validate L3 HA min/max _l3_agents_per_router

The below missing validation is added for L3 HA
max_l3_agents_per_router >= min_l3_agents_per_router

Closes-bug: #1400311
Change-Id: I1d548b9a0a04c8855ada42206c2a333597c2c85b
Merge "validate L3 HA min/max _l3_agents_per_router"