Add HashiCorp Nomad provider (#483)

* provider: adding Nomad provider

* updating CONTRIBUTING.md with Nomad provider

* updated README.md by adding the Nomad provider

* fix typo

* adding nomad/api and nomad/testutil deps

* adding Nomad binary dependency for provider tests

* fixed the nomad binary download command step and added tolerations to the nomad provider.

* adding nomad provider demo gif

* adding my name to authors

* adding two missing go-rootcerts files after dep ensure

* delete pod comment

providers/nomad/pods/nginx-pod.yaml

@@ -0,0 +1,22 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: nginx
+spec:
+  containers:
+  - image: nginx
+    imagePullPolicy: Always
+    name: nginx
+    ports:
+    - containerPort: 80
+    - containerPort: 443
+  dnsPolicy: ClusterFirst
+  nodeSelector:
+    kubernetes.io/role: agent
+    beta.kubernetes.io/os: linux
+    type: virtual-kubelet
+  tolerations:
+  - key: virtual-kubelet.io/provider
+    operator: Exists
+  - key: hashicorp.com/nomad
+    effect: NoSchedule