Bring ksonnet update into sync with ksonnet.next spec

[hausdorff]

The behavior of the ksonnet update command is specified in the ksonnet.next design doc from August 2017.

Bringing the command to specification implies the following work items:

• Completed in Implement init; bring synchronization primitives to spec #88. Update help documentation.
• Decide on the name. The community seems to believe apply is clearer, so it seems like this is probably the leading contender.
  • Completed in Deprecate update subcommand; introduce apply #105. Change update to apply. Deprecate update. Keep for two release cycles.
• Completed in Implement init; bring synchronization primitives to spec #88. Change update to:
  1. Expand and apply everything in the components/ directory, if the --env flag is passed.
  2. Do the same thing it currently does if the --file flag is passed.

[hausdorff]

Update, we've decided on the name. It will be apply. But we're going to land the prototype changes first. The plan is:

• Mark update as deprecated.
• Keep it for 2 minor releases.
• Transition to apply.