acs-engine needs at least 9 bits for custom VNET

[nakah]

Is this a request for help?:
No

Is this an ISSUE or FEATURE REQUEST? (choose one):
ISSUE

What version of acs-engine?:
0.9.3

Orchestrator and version (e.g. Kubernetes, DC/OS, Swarm)
Kubernetes

What happened:
I tried to deploy a Kubernetes cluster on a custom VNET within a /24 subnet. However, during validation I received the follwoing error : "must reserve at least 9 bits for node" which is here in the code.
The network team that I'm working with can't create a /23 subnet for each of our environment (that's too many reserved IP's) as Azure VNETs have a 4000 IP limit

What you expected to happen:
The cluster should have been created without any issue

How to reproduce it (as minimally and precisely as possible):
Deploy a K8S cluster on a custom VNET with a /24 subnet

Anything else we need to know:

[jackfrancis]

I would assume this is due to the fact that restricting a cluster to ~250 IP addresses is an edge case that we can't currently manage, and don't want customers to get themselves into: a cluster that needs to scale, but has no more IPs to allocate to nodes.

@JackQuincy @anhowe was that the thinking behind requiring a network with 2^9-ish allocatable IPs for a cluster? Are we willing to revisit this?

[nakah]

Just to precise that I'm not using any network policy, so VNET IP's are for nodes only. Thus I don't think that clusters with less thant 250 nodes is an edge case