Move instructions on startup with known_hosts into site/helm/get_started

What does this MR/Commit do?

- Moves docs that are confusing in `site/standalone` into `site/helm`
and also charts/README.md

Why is this MR needed?

- To avoid confusion about the appropriate time to use `helm install`
but to continue to offer documentation on how to set the KNOWN_HOSTS
on bootstrapping the helm-operator

site/helm/get-started.md

@@ -59,23 +59,33 @@ Add the Flux repository of Weaveworks:
 helm repo add weaveworks https://weaveworks.github.io/flux
 ```
 
-In this next step you install Weave Flux using `helm`. Simply:
+In this next step you install Weave Flux using `helm`. Simply
 
  1. Fork [flux-helm-test](https://github.com/weaveworks/flux-helm-test)
     on Github and
- 1. install Weave Flux and its Helm Operator by specifying your fork
+ 1. Install Weave Flux and its Helm Operator by specifying your fork
     URL:
 
-    *Just make sure you replace `YOURUSER` with your GitHub username
-    in the command below:*
-    ```sh
-    helm install --name flux \
-    --set helmOperator.create=true \
-    --set git.url=ssh://[email protected]/YOURUSER/flux-helm-test \
-    --set git.chartsPath=charts \
-    --namespace flux \
-    weaveworks/flux
-    ```
+      *Just make sure you replace `YOURUSER` with your GitHub username
+      in the command below:*
+
+    - Using a public git server from `bitbucket.com`, `github.com` or `gitlab.com`:
+
+      ```sh
+      helm install --name flux \
+      --set helmOperator.create=true \
+      --set git.url=ssh://[email protected]/YOURUSER/flux-helm-test \
+      --set git.chartsPath=charts \
+      --namespace flux \
+      weaveworks/flux
+      ```
+
+    - Using a private git server:
+
+      When deploying from a private repo, the known_hosts of the git server needs 
+      to be configured into a kubernetes configmap so that `StrictHostKeyChecking` is respected.
+      See [chart/flux/README.md](https://github.com/weaveworks/flux/blob/master/chart/flux/README.md#to-install-flux-with-the-helm-operator-and-a-private-git-repository)
+      for further installation instructions in this case.
 
 Allow some time for all containers to get up and running. If you're
 impatient, run the following command and see the pod creation

site/standalone/setup.md

@@ -136,21 +136,3 @@ manifest.
 You will need to explicitly tell fluxd to use that service account by
 uncommenting and possible adapting the line `# serviceAccountName:
 flux` in the file `fluxd-deployment.yaml` before applying it.
-
-It is also possible to create the known_hosts keys at the same time 
-as doing a `helm install` of `flux` and the `flux-helm-operator`. Line
-breaks can be used to separate one key from the next. See example below:
-
-```bash
-KNOWN_HOSTS='domain ssh-rsa line1
-domain ecdsa-sha2-line2
-domain ssh-ed25519 line3'
-
-helm install \
---name flux \
---set helmOperator.create=true \
---set git.url="ssh://[email protected]:weaveworks/flux-helm-test.git" \
---set-string ssh.known_hosts="${KNOWN_HOSTS}" \
---namespace flux \
-chart/flux
-```