Stop wrapping and returning nil

[nak3]

Description of changes:

Currently err in arg of the second errors.Wrapf() in getENIaddresses() will never
receive error except for nil value.

• L#584 checks err != nil and return, hence L#595 will never get err except for nil.
  

  amazon-vpc-cni-k8s/ipamd/ipamd.go

  Lines 583 to 595 in 0736312

  	ec2Addrs, _, err := c.awsClient.DescribeENI(eni)
  	if err != nil {
  	return nil, "", errors.Wrapf(err, "fail to find eni addresses for eni %s", eni)
  	}
  	for _, ec2Addr := range ec2Addrs {
  	if aws.BoolValue(ec2Addr.Primary) {
  	eniPrimaryIP := aws.StringValue(ec2Addr.PrivateIpAddress)
  	return ec2Addrs, eniPrimaryIP, nil
  	}
  	}
  	return nil, "", errors.Wrapf(err, "faind to find eni's primary address for eni %s", eni)

When errors.Wrapf() is called with nil in first arg, the new message
is not wrapped. To make matters worse, getENIaddresses() returns
error as nil so the logic of code after the method could be collapsed.

Hence, this patch uses errors.Errorf() instead errors.Wrapf().

Also, current unit tests for ipamd use two primary IPs for one
ENI. This patch fixes it as well.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[nak3]

Here is a small test code when errors.Wrap gets nil.

package main

import (
	"fmt"
	"github.com/pkg/errors"
)

func main() {
	var err error

	err = nil
	err = errors.Wrap(err, "I want to check this error")

	fmt.Printf("error w/ nil:  %v\n", err)
	if err == nil {
		fmt.Printf("error is still nil.\n")
	}

	/* OUTPUT
	$ go run a.go
	error w/ nil:  <nil>
	error is still nil.
	*/
}

[mogren]

Thanks, LGTM.