Change tryAssignIPs to assign up to configured WARM_IP_TARGET (#1279)

Previously tryAssignIPs would always allocate all available IPs on the
ENI regardless of the number of IPs the node would want. A great
showcase of this behavior is on node bootstrap. Assuming we have a node
with maxIPsPerENI=30 and WARM_IP_TARGET=5, the following happens:
- Node starts with single IP
- tryAssignIPs allocates an additional 29 IPs
- decreaseIPPool will release 29-5=24 IPs

After the change this would look like:
- Node starts with a single IP
- tryAssignIPs allocates an additional 5 IPs

So in this very common flow we cut out an entire AWS API call per node.
In addition this behavior exhibits itself any time we need to scale up
IPs as tryAssignIPs always attempts to over-allocate IPs -- which then
just need to be released almost immediately.

Fixes #1272

pkg/ipamd/ipamd.go

@@ -16,6 +16,7 @@ package ipamd
 import (
 	"context"
 	"fmt"
+	"math"
 	"net"
 	"os"
 	"strconv"
@@ -746,12 +747,19 @@ func (c *IPAMContext) tryAssignIPs() (increasedPool bool, err error) {
 		return false, nil
 	}
 
+	// If WARM_IP_TARGET is set we only want to allocate up to that target
+	// to avoid overallocating and releasing
+	toAllocate := c.maxIPsPerENI
+	if warmIPTargetDefined {
+		toAllocate = short
+	}
+
 	// Find an ENI where we can add more IPs
 	eni := c.dataStore.GetENINeedsIP(c.maxIPsPerENI, c.useCustomNetworking)
 	if eni != nil && len(eni.IPv4Addresses) < c.maxIPsPerENI {
 		currentNumberOfAllocatedIPs := len(eni.IPv4Addresses)
 		// Try to allocate all available IPs for this ENI
-		err = c.awsClient.AllocIPAddresses(eni.ID, c.maxIPsPerENI-currentNumberOfAllocatedIPs)
+		err = c.awsClient.AllocIPAddresses(eni.ID, int(math.Min(float64(c.maxIPsPerENI-currentNumberOfAllocatedIPs), float64(toAllocate))))
 		if err != nil {
 			log.Warnf("failed to allocate all available IP addresses on ENI %s, err: %v", eni.ID, err)
 			// Try to just get one more IP