l3_agent: avoid name conflict with context

module name, context, conflicts with argument name in many place in
l3_agent. In order to avoid such conflict, import context as n_context
following Neutron practice.

Change-Id: Ic3754818f84064d2c8da04914826fc912437b2f0
Closes-Bug: #1381900
Merge "l3_agent: avoid name conflict with context"