Add an e2e host-local plugin testcase, which requires being able
to pass the datadir into the plugin so we can erase it later.
We're not always guaranteed to have access to the default data
dir location, plus it should probably be configurable anyway.
This changes the ip allocation logic to round robin. Before this, host-local IPAM searched for available IPs from start of subnet. Hence it tends to allocate IPs that had been used recently. This is not ideal since it may cause collisions.
This adds basic plugins.
"main" types: veth, bridge, macvlan
"ipam" type: host-local
The code has been ported over from github.com/coreos/rkt project
and adapted to fit the CNI spec.
