Introduce a minimum target for ENI IPs (aws#612)

* Introduce a minimum target for ENI IPs

Adds a MINIMUM_IP_TARGET environment variable to inform the AWS CNI that a
particular number of total IPs is anticipated for use with a particular node.
This is useful to ensure a sufficient supply of IPs on a node up-front without
the 2x IP usage overhead of setting WARM_IP_TARGET to the same value.

* Fix multiple incorrect references to MINIMUM_IP_TARGET

Co-Authored-By: Ed Morley <501702+edmorley@users.noreply.github.com>

README.md

@@ -243,6 +243,24 @@ EC2 API and that might cause throttling of the requests. It is strongly suggeste
 If both `WARM_IP_TARGET` and `MINIMUM_IP_TARGET` are set, `ipamD` will attempt to meet both constraints.
 This environment variable overrides `WARM_ENI_TARGET` behavior.
 
+`MINIMUM_IP_TARGET`
+Type: Integer
+Default: None
+
+Specifies the number of total IP addresses that the `ipamD` daemon should attempt to allocate for pod assignment on the node.
+`MINIMUM_IP_TARGET` behaves identically to `WARM_IP_TARGET` except that instead of setting a target number of free IP
+addresses to keep available at all times, it sets a target number for a floor on how many total IP addresses are allocated.
+
+`MINIMUM_IP_TARGET` is for pre-scaling, `WARM_IP_TARGET` is for dynamic scaling. For example, suppose a cluster has an
+expected pod density of approximately 30 pods per node. If `WARM_IP_TARGET` is set to 30 to ensure there are enough IPs
+allocated up front by the CNI, then 30 pods are deployed to the node, the CNI will allocate an additional 30 IPs, for
+a total of 60, accelerating IP exhaustion in the relevant subnets. If instead `MINIMUM_IP_TARGET` is set to 30 and
+`WARM_IP_TARGET` to 2, after the 30 pods are deployed the CNI would allocate an additional 2 IPs. This still provides
+elasticity, but uses roughly half as many IPs as using WARM_IP_TARGET alone (32 IPs vs 60 IPs).
+
+This also improves reliability of the EKS cluster by reducing the number of calls necessary to allocate or deallocate
+private IPs, which may be throttled, especially at scaling-related times.
+
 ---
 
 `MINIMUM_IP_TARGET` (Since v1.6.0)