Fix --extra-config for scheduler/controllerManager by removing hardcoded values

[priyawadhwa]

Now, users can specify additional flags to scheduler/controllerManager without overriding the place in the config where we set leader-elect=false

Verified that this works locally

Closes #9134

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: priyawadhwa

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[priyawadhwa]

/ok-to-test

[minikube-pr-bot]

kvm2 Driver
Times for minikube: 67.2s 63.3s 62.6s
Average time for minikube: 64.3s

Times for Minikube (PR 9136): 62.0s 65.8s 64.6s
Average time for Minikube (PR 9136): 64.1s

Averages Time Per Log

+--------------------------------+----------+--------------------+
|              LOG               | MINIKUBE | MINIKUBE (PR 9136) |
+--------------------------------+----------+--------------------+
| * minikube v1.12.3 on Debian   | 0.1s     | 0.1s               |
|                           9.11 |          |                    |
| * Using the kvm2 driver based  | 0.0s     | 0.0s               |
| on user configuration          |          |                    |
| * Starting control plane node  | 0.0s     | 0.0s               |
| minikube in cluster minikube   |          |                    |
| * Creating kvm2 VM (CPUs=2,    | 38.1s    | 39.0s              |
| Memory=3700MB, Disk=20000MB)   |          |                    |
| ...                            |          |                    |
| * Preparing Kubernetes v1.19.0 | 24.2s    | 22.9s              |
| on Docker 19.03.12 ...         |          |                    |
| * Verifying Kubernetes         | 1.6s     | 1.6s               |
| components...                  |          |                    |
| * Enabled addons:              | 0.2s     | 0.4s               |
| default-storageclass,          |          |                    |
| storage-provisioner            |          |                    |
| * Done! kubectl is now         | 0.1s     | 0.1s               |
| configured to use "minikube"   |          |                    |
|                                | 0.0s     | 0.0s               |
+--------------------------------+----------+--------------------+

docker Driver
Times for minikube: 28.1s 30.3s 28.8s
Average time for minikube: 29.1s

Times for Minikube (PR 9136): 30.4s 29.6s 28.7s
Average time for Minikube (PR 9136): 29.6s

Averages Time Per Log

+--------------------------------+----------+--------------------+
|              LOG               | MINIKUBE | MINIKUBE (PR 9136) |
+--------------------------------+----------+--------------------+
| * minikube v1.12.3 on Debian   | 0.1s     | 0.1s               |
|                           9.11 |          |                    |
| * Using the docker driver      | 0.1s     | 0.1s               |
| based on user configuration    |          |                    |
| * Starting control plane node  | 0.1s     | 0.1s               |
| minikube in cluster minikube   |          |                    |
| * Creating docker container    | 8.7s     | 8.7s               |
| (CPUs=2, Memory=3700MB) ...    |          |                    |
| * Preparing Kubernetes v1.19.0 | 19.2s    | 19.7s              |
| on Docker 19.03.8 ...          |          |                    |
| * Verifying Kubernetes         | 0.9s     | 0.8s               |
| components...                  |          |                    |
| * Enabled addons:              | 0.0s     | 0.0s               |
| default-storageclass,          |          |                    |
| storage-provisioner            |          |                    |
| * Done! kubectl is now         | 0.1s     | 0.1s               |
| configured to use "minikube"   |          |                    |
|                                | 0.0s     | 0.0s               |
+--------------------------------+----------+--------------------+

[tstromberg]

Thank you!