This adds support to specify node selectors on Pod node labels.
They are used by the k8s scheduler to place a Pod on specific nodes with
corresponding labels.
This allows to place a build node/Pod on k8s nodes with certain
capabilities (e.g. storage types, number of CPU cores, etc.)
Change-Id: Ic00a84181c8ef66189e4259ef6434dc62b81c3c6
19be1a2e26