Reimplement L3Agent as router's property

We implemented Neutron's L3Agent resource as Heat's RouterL3Agent resource at
once.

  https://review.openstack.org/#/c/61388/

However, that model can change the router_id when doing stack-update.
The bug 1272254 was caused by that model.

We discussed about how to resolve the bug 1272254 with the Heat community.
The conclusion is to remove RouterL3Agent, then to reimplement agent_id as
router's property. This model can't change router_id. So its model can resolve
the bug.

Closes-bug: #1272254
Change-Id: Id9f75920ec46ec32f3e41bdb97f89a7fd09490ec
Merge "Reimplement L3Agent as router's property"