Include system affinity for when ASO runs on AKS

[theunrepentantgeek]

Describe the improvement

A suggestion made via email:

My suggestion to improve ASO would be to include the following node affinity block that would target system nodepools on any AKS cluster:

affinity:
    nodeAffinity:
        requiredDuringSchedulingIgnoredDuringExecution:
          nodeSelectorTerms:
          - matchExpressions:
            - key: kubernetes.azure.com/mode
              operator: In
              values:
                - system

According to @matthchr we shouldn't use required because it ASO is supported on non AKS clusters, but there's a preferredDuringSchedulingIgnoredDuringExecution we could use too.

[matthchr]

What do we have set for mem/cpu requirements - are they too low?

[matthchr]

We may need a negative assertion rather than positive: "Don't schedule on the user pool", rather than "must schedule on the system pool"

and wahtever we do should probably be put in a Helm "if" block.