Before this change, the podCIDRs map contained both cidrs and ips
depending on which code path entered a container into it.
Specifically, SetUpPod would enter a CIDR while GetPodNetworkStatus
would enter an IP.
This normalizes both of them to always enter just IP addresses.
This also removes the now-redundant cidr parsing that was used to get
the ip before
When the IP isn't in the internal map, GetPodNetworkStatus() needs
to call the execer for the 'nsenter' program. That means the execer
needs to be !nil, which it wasn't before.
