Infrakit release v0.3.0

This release contains bug fixes, API and schema changes, as well as a major reworking of the Swarm flavor plugin.

If you are vendoring Infrakit packages in your project, code change will be necessary because interface signatures have changed (see #371)

Highlights:

• New types introduced + SPI method signature changes to use plugin.Name and types.Any (#362, #363, #371)
• A new Label() method has been added to the Instance Plugin SPI (#371).
  • Updated implementations for file, terraform plugins
  • Added tests for file, terraform plugins.
• Multiple groups can now be specified in a JSON config for the manager (#368)
  • New commands: infrakit manager commit / inspect
  • Manager accepts JSON template, performs string interpolation.
• Plugin management improvement
  • New commands (infrakit plugin start / stop) (#367)
  • Configuration JSON schema changed to support different kinds of executors (e.g. os, docker-run). (#374)
• Templating engine improvements (#366, #373)
• Change to plugin handshake protocol. Handshake for version checking is performed at connection time, not per-call (#369) and as early as possible.
• Vagrant plugin improvement - real-time streaming of vagrant logging (#375)
• Swarm flavor plugin revamp (#376)
  • Instance startup init script now programmable via templates
  • Changed config schema and eliminated dependency on Docker connection parameters on plugin startup via CLI flags
  • A single commit of a configuration JSON template now provision multiple groups (e.g. managers and workers)