Make sure floating IPs + gateways must be on external nets

Make sure floating IPs  + gateways must be on external nets

bug #1042030

- adds admin-writable, world-readable router:external attribute to
the network object if L3 extension is loaded.
- prevents floating ips from being created unless network is external
- shortens L3 extensions alias from 'os-quantum-router' to 'router' to
make attribute extensions more readable.

WIP:
- Need to add policy logic so non-admin users can always see external
networks without requiring that these networks are shared (since VMs can
always create ports on shared networks, but provider may want to have
externals networks that VMs cannot directly plug into.

Random clean-up:
- prevent delete_network in plugins from implying it returns something
- modify extensions.py so that exceptions during calls to
get_extended_resources() will actually be logged if unexpected.
- unset executable bit on test_iptables_manager.py to make sure tox
actually runs it.

Change-Id: I5bbf063927b93458da7cb467d9ad5c92ebabbbf7