Allow to deploy router as DaemonSet (#1342)

ReplicaSet generated by Deployment distributes pods to nodes based
on nodes resource usage, which means in some cases the newly created
pods may be assigned to only a few nodes. The requests will go to the
same node if there multiple pods on it and may increase the node
resource usage and overall request latency.

To solve the problem mentioned above, this PR allows users to deploy
the router as DaemonSet so that we can distribute requests across all
nodes for better workload distribution and lower latency.

charts/README.md

@@ -5,7 +5,7 @@
 
 ## Prerequisites
 
-- Kubernetes 1.6 or later
+- Kubernetes 1.9 or later
 
 
 ## Helm charts
@@ -65,6 +65,7 @@ Parameter | Description | Default
 `prometheusDeploy` | Set to true if prometheus needs to be deployed along with fission | `true` in `fission-all`, `false` in `fission-core`
 `canaryDeployment.enabled` | Set to true if you need canary deployment feature | `true` in `fission-all`, `false` in `fission-core`
 `extraCoreComponentPodConfig` | Extend the container specs for the core fission pods. Can be used to add things like affinty/tolerations/nodeSelectors/etc. | None
+`router.deployAsDaemonSet` | Deploy router as DaemonSet instead of Deployment | `false`
 `router.svcAddressMaxRetries` | Max retries times for router to retry on a certain service URL returns from cache/executor | `5`
 `router.svcAddressUpdateTimeout` | The length of update lock expiry time for router to get a service URL returns from executor | `30`
 `router.svcAnnotations` | Annotations for router service | None