Change router's prop name agent_id to l3_agent_id

Currently, the OS::Neutron::Router supports associating the L3Agent via
"agent_id" property.

However, Neutron supports a few kind of agents(L3, DHCP and etc).
The property name of "agent_id" is little bit ambiguous.

It's nice to change property name "agent_id" to "l3_agent_id".

Change-Id: Ie3c90d703df81bbf9d541a7d7c08641eed9c8e2a
Closes-bug: #1296608
Merge "Change router's prop name agent_id to l3_agent_id"