Previously, minikube has been shipped with the default CNI config
(/etc/cni/net.d/k8s.conf) in its rootfs. This complicated a lot
when using a custom CNI plugin, as the default config was picked
by kubelet before the custom CNI plugin has installed its own CNI
config. So, the end result was that some Pods were attached to a
network defined in the default config, and some got managed by
the custom plugin.

This commit introduces the flag "--enable-default-cni" to
"minikube start" to trigger the provisioning of the default CNI
config.
Signed-off-by: NMartynas Pumputis <m@lambda.lt>