when ipip=CrossSubnet, ipip is always used, even on the same subnet

[George Kraft]

When the calico charm is configured with ipip=CrossSubnet, ipip encapsulation is always used, even between workers that are on the same subnet.

Expected behavior is for ipip encapsulation to be used only between workers that are on different subnets.

Observed with Charmed Kubernetes 1.18+ck2 and cs:~containers/calico-741.

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Reproduction steps
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1. Deploy cs:~containers/kubernetes-calico. Ensure that at least two kubernetes-worker units are placed on the same subnet.

2. Configure the calico charm with ipip=CrossSubnet

Once the deployment has settled, run `ip route` on worker units to see where IPIP encapsulation is being used. If a route between workers on the same subnet goes through tunl0, then it is using IPIP encapsulation for that route, and that is a bug.

[Narinder Gupta (narindergupta)]

We observed the similar issue on 1.17 version as well with calico cs:~containers/calico-741 on bionic.