Switch back to single Flannel manifest for all platforms

[geerlingguy]

The upstream Flannel repo resolved the issue that required the Flannel manifest to be installed differently on arm vs amd64 architectures. See: flannel-io/flannel#989

So I would like to switch back to just using the default Flannel manifest (https://raw.githubusercontent.com/coreos/flannel/master/Documentation/kube-flannel.yml), and drop the custom stuff I had set up to swap in 'arm' for 'amd64'.

[geerlingguy]

See also: flannel-io/flannel#663

[geerlingguy]

I was going to drop the Vagrant manifest override, but I need that to keep the - --iface=enp0s8 arg for flannel... that's what was causing issues with DNS inside one of the containers.