GCP generated keys should take a name parameter

[displague]

The GCP key generating script https://github.com/equinix/terraform-metal-anthos-on-vsphere/blob/main/anthos/create_service_accounts.sh creates keys with a fixed name. This prevents other users sharing that project from generating keys themselves.

The keys must then be shared among team members.

We can provide a better experience by taking an optional name prefix/suffix.

Similar to equinix/terraform-equinix-metal-anthos-on-baremetal#72