gcp: Enable nested virt by default

Per comment from a GCP engineer, this is basically saying our
OS supports KVM, which it does.

We're enabling this only in the FCOS pipeline right now but
it makes sense to enable for RHCOS too.

See openshift/installer#3808

src/cosalib/gcp.py

@@ -6,6 +6,10 @@
     stop_after_attempt
 )
 
+# https://github.com/coreos/coreos-assembler/pull/1477#issuecomment-634275570
+DEFAULT_LICENSES = [
+    "https://compute.googleapis.com/compute/v1/projects/vm-options/global/licenses/enable-vmx"
+]
 
 # This is the naming rule used by GCP and is used to check image
 # names during upload. See:
@@ -69,9 +73,8 @@ def gcp_run_ore(build, args):
         ore_upload_cmd.extend(['--public'])
     if not args.create_image:
         ore_upload_cmd.extend(['--create-image=false'])
-    if args.license:
-        for license in args.license:
-            ore_upload_cmd.extend(['--license', license])
+    for license in args.license or DEFAULT_LICENSES:
+        ore_upload_cmd.extend(['--license', license])
     run_verbose(ore_upload_cmd)
 
     # Run deprecate image to deprecate if requested