Support assigning Pods to Nodes using nodeSelector

[dcasavant]

In some Kubernetes environments, you may have configured certain Nodes in your Kubernetes cluster with different attributes, such as persistent storage. You may want to use the nodeSelector pattern to assign Pods of a tier to run on particular node(s).

This change adds the ability to use nodeSelector as a block under a particular tier.

Example use cases:

1. Assign Stream tier Pods to Kubernetes Nodes with durable disks attached.
2. Assign all Pega Pods to Kubernetes Nodes with extra memory.

To specify a nodeSelector, add the option under a particular tier, such as in the example below:

tier:
- name: "stream"
  nodeType: "Stream"

  nodeSelector:
    disktype: ssd

[taz-mon]

hi Dave - I'll approve so you can move this forward, but I'd suggest changes here. It'd be useful to use a little more direct language here and include the context of the node selector label, since the linked article uses the label terminology? This is what I'm thinking:
Pega supports configuring certain nodes in your Kubernetes cluster with a label to identify its attributes, such as persistent storage. For such configurations, use the Pega Helm chart nodeSelector property to assign pods in a tier to run on particular nodes with a specified label. For more information about assigning Pods to Nodes including how to configure your Nodes with labels, see the Kubernetes documentation on nodeSelector.