Allow the driver choice to persist from/to the profile

Just so after we specify it once, we don't have to keep specifying it.
It might make sense to do this for the other options as well.

Also, by deleting the vm *after* setting the profile, and not passing
--profile, we avoid a weird error that forced us to install twice.

Author: jcrossley3

etc/scripts/install-on-minikube.sh

@@ -19,25 +19,26 @@ KUBERNETES_VERSION=${KUBERNETES_VERSION:-v1.12.0}
 MEMORY=${MEMORY:-8192}
 CPUS=${CPUS:-4}
 DISK_SIZE=${DISK_SIZE:-50g}
-VM_DRIVER=${VM_DRIVER:-virtualbox}
-
-# blow away everything in the knative profile
-minikube delete --profile knative
+VM_DRIVER=${VM_DRIVER:-$(minikube config get vm-driver 2>/dev/null || echo "virtualbox")}
 
 # configure knative profile
 minikube profile knative
 minikube config set kubernetes-version ${KUBERNETES_VERSION}
 minikube config set memory ${MEMORY}
 minikube config set cpus ${CPUS}
 minikube config set disk-size ${DISK_SIZE}
+minikube config set vm-driver ${VM_DRIVER}
+
+# blow away everything in the knative profile
+minikube delete
 
 # Start minikube
-minikube start -p knative --vm-driver $VM_DRIVER --extra-config=apiserver.enable-admission-plugins="LimitRanger,NamespaceExists,NamespaceLifecycle,ResourceQuota,ServiceAccount,DefaultStorageClass,MutatingAdmissionWebhook"
+minikube start -p knative --extra-config=apiserver.enable-admission-plugins="LimitRanger,NamespaceExists,NamespaceLifecycle,ResourceQuota,ServiceAccount,DefaultStorageClass,MutatingAdmissionWebhook"
 
 if [ $? -eq 0 ]; then
   DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null && pwd )"
   "$DIR/install.sh" -q
 else
   echo "Failed to start minikube!"
-  exit $exit_status
+  exit -1
 fi