fix: make master_ipv4_cidr_block optional for autopilot

[cloud-pharaoh]

Fixing a bug with Autopilot Private cluster submodule

The default value provided in the module makes it not possible to create multiple clusters in the same VPC. This was likely added to cover for a provider bug that required master_ipv4_cidr_block to be set even for autopilot clusters but it was fixed days after the module additions for autopilot private were merged.

• Docs :

Note: In an Autopilot cluster, the --master-ipv4-cidr parameter is assigned a 
subnet within the 172.16.0.0/16 range by default. 
This range allows for various subnet sizes.

• Provider docs:

master_ipv4_cidr_block (Optional) - The IP range in CIDR notation to use for the hosted master network. 
This range will be used for assigning private IP addresses to the cluster master(s) and the ILB VIP. 
This range must not overlap with any other ranges in use within the cluster's network, 
and it must be a /28 subnet. See Private Cluster Limitations for more details. 
This field only applies to private clusters, when enable_private_nodes is true.

• Module docs:

master_ipv4_cidr_block	(Beta) The IP range in CIDR notation to use for the hosted master network	string	"10.0.0.0/28"

[bharathkkb]

Thanks for the PR @cloud-pharaoh
Could you make the changes to autogen and run make build to generate the modules.

[cloud-pharaoh]

Thanks @bharathkkb! Looks like I haven't contributed in so long. Things have changed a lot :)

[gtsorbo]

/gcbrun

[cloud-pharaoh]

@gtsorbo looks like an unrelated error and just needs to be rerun?

Step #37 - "verify simple-zonal-local":     jsonresult.go:42: Error parsing output, invalid json: E0314 17:30:29.310339     415 memcache.go:287] couldn't get resource list for metrics.k8s.io/v1beta1: the server is currently unable to handle the request