Tolerate resource lookup errors for quantum FnGetAtt.

Fixes bug #1180293

Change-Id: I5384456bbbf53558036573c983be0b65e8883612
Tests for Router, RouterInterface, RouterGateway.

quantum package now has over 94% test coverage.

Change-Id: I82b4635e4554e5b94eaac70bd1cc4b9249863dc9
Exclude build/ from flake8 checks

Change-Id: Ie7b19f8488230e8538720b96c73b49453c6a3efe
Merge "Exclude build/ from flake8 checks"