Update kube.libsonnet for newer (1.8+) k8s

[anguslees]

Many small diffs.

New features:

• Add parseOctal helper function (eg useful for volume mode).
• Add toUpper and toLower.
• Change : to - when preparing default name label (eg appears in system object names but is not safe in label keys).
• Add host and host_colon_port helpers to Service.
• Set container imagePullPolicy default explicitly (following usual k8s default logic). In particular this means the server-side value will be updated if the jsonnet image is updated to/from :latest.
• Add PodDisruptionBudget.
• "First container" logic now defaults to the container named default rather than requiring a value for default_container in the multi-container case. default_container continues to override.
• Add container initContainers_, similar to containers_.
• HostPathVolume now takes an optional type for convenience and brevity.
• ResourceFieldRef now takes divisor as an optional arg rather than object field, for brevity.
• Set selector explicitly for Deployment, StatefulSet, DaemonSet.
• Provide an explicit RollingUpdate strategy for StatefulSet and DaemonSet.
• Add jsonnet assert for invalid (relative) paths in Ingress.

Changes to previous behaviour:

• Change default _Object name label to follow updates to metadata.name rather than continue to use original _Object arg.
• Change default Service targetPort to use target's (numeric) containerPort rather than (string) name. containerPort is required, whereas name is optional.
• Deployment now uses apps/v1beta2, and non-default revisionHistoryLimit is removed (10 is now the server default)
• StatefulSet now uses apps/v1beta2.
• DaemonSet now uses apps/v1beta2.
• Slightly change workaround for StatefulSet non-semantic diff bug, to take advantage of std.prune. May require explicitly restoring some of the previous "avoid-omitempty" workaround values for existing StatefulSets to avoid server-side validation error on updates.
• Changed JobSpec arg->override to match style of similar PodSpec.
• CustomResourceDefinition changed to take more useful "gvk" args.

[anguslees]

Happy to break any/all of this into separate PRs if you would rather consider portions separately.