Acceptance criteria:

• When users specify osdisk.EncryptionKey in the gcp machine pool, capi provisioned machines are created using those encryption keys.

The gcpmachine (defined at
https://github.com/kubernetes-sigs/cluster-api-provider-gcp/blob/245e21e741b565d791070bfae9b1e1920e1ee5e5/api/v1beta1/gcpmachine_types.go)
includes RootDiskType and RootDeviceSize, but it does not provide for DiskEncryption.

In order to be compatible with the GCP MachineAPI - https://github.com/openshift/installer/blob/master/pkg/types/gcp/machinepools.go#L78 - a Disk EncryptionKey field should be added to the GCP machine.