worker nodes for aws k8s cluster

[alanbchristie]

• Adds SQUONK_POD_NODE_AFFINITY_VALUE (default 'worker')
• Adds SQUONK_NF_QUEUE_SIZE (default 100)
• Pods now only run on worker nodes
• Fixed for AsynHttpClient test
• DockerRunner now always pulls image - rather than checking whether it exists locally
• Tested on STAGING

[shortcut-integration[bot]]

This pull request has been linked to Clubhouse Story #1622: Worker nodes for AWS K8S cluster.