Add 'secure-boot-for-shielded-vms' flag to expose secureboot on GCP

[JakobGray]

Issue: https://issues.redhat.com/browse/OCM-4974

• Adds --secure-boot-for-shielded-vms flag for enabling secure boot on GCP clusters.
• Adds secureBoot field to cluster describe output (only when enabled)
• Adds interactive prompt for secureBoot after multi-az (only for GCP)

Interactive view:

? Multiple AZ: No
? Secure Boot enables the use of Shielded VMs in the Google Cloud Platform.
? Secure boot support for Shielded VMs: (y/N) 

Create cluster help view:

Usage:
  ocm create cluster [flags] NAME

Flags:
      --secure-boot-for-shielded-vms                          Secure Boot enables the use of Shielded VMs in the Google Cloud Platform.

Cluster describe view:

ID:			27v4ri1ot49embn20k3qot1fspjoas6e
Multi-az:		false
SecureBoot:             true
CCS:			true

[renan-campos]

/lgtm

[ckandag]

/lgtm

[hunterkepley]

/lgtm