Update Kubernetes deployment instructions to point to 2.6 and some tweaks

https://docs.couchbase.com/sync-gateway/2.6/kubernetes/deploy-cluster.html

Pls. update with attached deployment yaml files (both regular and import nodes)

Changes

• Sync docker image is now pointing to 2.6
• Added an "anti-affinity" entry . Please add a bullet to explain the following
  • The "anti-affinity" setting will not schedule two Sync Gateway pods on the same node. This setting is recommended to enable high availability and to prevent "noisy neighbor" issues. 
  • If you are on a test/development environment , you can remove this property to have all nodes on a single node.