Remove external IPs from GCP worker nodes

[rfranzke]

With gardener-attic/gardener-extensions#379 and gardener-attic/gardener-extensions#398 Cloud NATs have been introduced for GCP shoots in order to remove the external IPs from the shoot worker nodes.

However, these changes were reverted with gardener-attic/gardener-extensions#405 because of instabilities.
More specifically, the main problem was that shoots which are deployed into the same VPC/network get one router and cloud NAT each. However, GCP has a hard quota limit here:

From https://cloud.google.com/router/quotas:

Cloud Routers per project Quotas Regardless of quota, each network is limited to five Cloud Routers per region. See limits below.
The following limits for Cloud Router apply to VPC networks. Unless otherwise stated, these limits cannot be increased.

Hence, the implementation with gardener-attic/gardener-extensions#379 and gardener-attic/gardener-extensions#398 does not work.

What can we do to circumvent the problem and to get rid of the external IPs for GCP worker nodes?

[vpnachev]

Can we close this issue ?
Or it will be kept open till the option for external IP is completely removed.

[rfranzke]

@zanetworker what's the timeline/progress on the plans proposed in gardener/gardener-extensions#411 ?

if a VPC already exists and a CloudRouter name is not provided, then VMs are created the old way (with a public IP, until a deadline which dictates the use of a CloudRouter).
In two months time, this adapting code will be removed.

[vpnachev]

@zanetworker please open another PR and adapt the documentation here https://github.com/gardener/gardener-extensions/blob/master/controllers/provider-gcp/docs/usage-as-end-user.md