install/k8s: add statefulset and endpoint test

[jasonodonnell]

The K8s installation code checks if the K8s Load Balancer has an external IP address then proceeds to connect via gRPC to bootstrap the server. In some K8s environments the load balancer gets an external IP long before the pod is ready to accept connections.

For example, pods in EKS consistently took 30 seconds or more to start after the service already had an external IP. This is due to the overhead of EBS volume provisioning to satisfy the persistent volume claim.

I've added additional checks so we can ensure everything is ready before attempting connection:

• verify the Statefulset and all its replicas are ready
• check the service for container endpoints to ensure routing is ready
• simple HTTP connection test to ensure we reach the pod.

Fixes #431.

[vercel]

@jasonodonnell is attempting to deploy a commit to the HashiCorp Team on Vercel.

A member of the Team first needs to authorize it.

[evanphx]

@jasonodonnell We're going to hold this until after release.

[jasonodonnell]

@evanphx Works for me!