Properly support neutron subnet `host_routes` on the router appliance.
Handle IPv6 DHCP allocations

The code for generating the dnsmasq config had a bad python string
format expression, causing an unhandled exception when any IPv6 subnet
had DHCP enabled.

Fixes DHC-2126

Change-Id: Iede9fe7179911bb6df520696a9b978a288d7a5d4
Merge pull request #49 from dhellmann/bug/dhc-2126

Handle IPv6 DHCP allocations
Refactor the `host_routes` logic a bit.
Make `_alter_route` return an explicit boolean to indicate success or failure.
Add a test for `route` call failures.
Merge pull request #50 from ryanpetrello/feature-support-host-routes

Properly support neutron subnet `host_routes` on the router appliance.
Only open the route shelve if the user has specified custom host routes.
Merge pull request #51 from ryanpetrello/master

Only open the route shelve if the user has specified custom host routes.