(untested) allow flannel interface selection

[jayunit100]

Im sure an ansible power-user can steal or modify this, but just wanted to put it up here before i forget :) . I'm still learning about flannel internals and not an ansible expert either, so any feedback on this is welcome, or feel to just borrow the idea and reimplement correctly. '

long story short : if using VMs, eth0 is your enemy !

[jayunit100]

ALSO, please confirm that eth0 is the right default. maybe there should be some if/else logic in stead which leaves it optionally empty?

[eparis]

I feel like this will break more people than it fixes, aka every single setup on real hardware that has no eth0 (since they moved to the crazy location based names, like enp4s11 or enp0s29f7u4u1, which my system has.)

Maybe better to leave flannel_iface unset in the default and do something like https://github.com/eparis/kubernetes-ansible/blob/master/roles/minion/templates/kubelet.j2#L22 in the template?

[jayunit100]

gotcha, done (patch updated)

[eparis]

still untested? even on your setup?

[jayunit100]

haven't testd it. my VMs are all maxed out right now. after i do an experiment i can test it in vagrant.

Maybe i can build a vagrantfile for easy testing, run the test, and just go ahead and commit it w/ this PR ?

After all, a setup this sophisticated needs/deserves a good Vagrantfile for verifying new commits.