Add TCP Middlewares support

docs/content/middlewares/http/retry.md

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+# Retry
+
+Retrying until it Succeeds
+{: .subtitle }
+
+<!--
+TODO: add schema
+-->
+
+The Retry middleware reissues requests a given number of times to a backend server if that server does not reply.
+As soon as the server answers, the middleware stops retrying, regardless of the response status.
+The Retry middleware has an optional configuration to enable an exponential backoff.
+
+## Configuration Examples
+
+```yaml tab="Docker"
+# Retry 4 times with exponential backoff
+labels:
+  - "traefik.http.middlewares.test-retry.retry.attempts=4"
+  - "traefik.http.middlewares.test-retry.retry.initialinterval=100ms"
+```
+
+```yaml tab="Kubernetes"
+# Retry 4 times with exponential backoff
+apiVersion: traefik.containo.us/v1alpha1
+kind: Middleware
+metadata:
+  name: test-retry
+spec:
+  retry:
+    attempts: 4
+    initialInterval: 100ms
+```
+
+```yaml tab="Consul Catalog"
+# Retry 4 times with exponential backoff
+- "traefik.http.middlewares.test-retry.retry.attempts=4"
+- "traefik.http.middlewares.test-retry.retry.initialinterval=100ms"
+```
+
+```json tab="Marathon"
+"labels": {
+  "traefik.http.middlewares.test-retry.retry.attempts": "4",
+  "traefik.http.middlewares.test-retry.retry.initialinterval": "100ms",
+}
+```
+
+```yaml tab="Rancher"
+# Retry 4 times with exponential backoff
+labels:
+  - "traefik.http.middlewares.test-retry.retry.attempts=4"
+  - "traefik.http.middlewares.test-retry.retry.initialinterval=100ms"
+```
+
+```toml tab="File (TOML)"
+# Retry 4 times with exponential backoff
+[http.middlewares]
+  [http.middlewares.test-retry.retry]
+    attempts = 4
+    initialInterval = "100ms"
+```
+
+```yaml tab="File (YAML)"
+# Retry 4 times with exponential backoff
+http:
+  middlewares:
+    test-retry:
+      retry:
+        attempts: 4
+        initialInterval: 100ms
+```
+
+## Configuration Options
+
+### `attempts`
+
+_mandatory_
+
+The `attempts` option defines how many times the request should be retried.
+
+### `initialInterval`
+
+The `initialInterval` option defines the first wait time in the exponential backoff series. The maximum interval is
+calculated as twice the `initialInterval`. If unspecified, requests will be retried immediately.
+
+The value of initialInterval should be provided in seconds or as a valid duration format, see [time.ParseDuration](https://golang.org/pkg/time/#ParseDuration).