A specific IP can now be requested via the environment variable CNI_ARGS, e.g.
`CNI_ARGS=ip=1.2.3.4`.
The plugin will try to reserve the specified IP.
If this is not successful the execution will fail.
This introduces a notion of a "meta" plugin. A meta plugin
is one that delegates the actual work of setting up the interface
to the main plugin. The meta plugin is used to select and dynamically
configure the main plugin. The sequence of events, is as follows:
Given netconf like:
{
"name": "mynet",
"type": "flannel",
"delegate": {
"type": "bridge"
}
}
flannel fills in values like "mtu", "ipam.subnet" and delegates to
"bridge" main plugin. "bridge" plugin will operate as usual, calling
into ipam module for IP assignment.
Delegate dictionary should not contain "name" field as it will be
filled in by the flannel plugin.
