Added new test cases.
Added new test cases.

Add two interfaces (e.g. eth0, eth1) to the same container.
Ensure each file now has ContainerID and ifname.
Delete one, ensure that the right file was deleted.

Add an interface using just ContainerID in the file.
Delete to verify we are still backwards compatible with any
files created using earlier verison of host-local plugin.

containernetworking/plugins,containernetworking/plugins