Refactor management of namespaces in the L3 Agent

Refactor management of namespaces in the L3 Agent

Creates classes representing the 3 types of namespaces handled by the L3 agent:
router, snat and fip.

The scope of this change is:

- Creation and deletion methods are provided for each namespace class
- Creation and deletion of router and snat namespaces are moved to the router
  classes. These namespaces are now members of the corresponding router class
- Invocation of Fip namespace creation and deletion is left in the agent, since
  the agent owns it
- A context manager is provided to move the namespaces (router and snat)
  cleanup code out of the agent

A follow up patchset will add methods to create and delete interfaces in the
namespaces. These methods are intended to be used by the router classes

Change-Id: I54b14e593ded6b2990d57a3ae9d598a699ae133e
Partially-Implements: bp restructure-l3-agent