Add build metadata to boxes and build artifacts.

[fnichol]

For each Packer template that is run via bin/bento, a JSON file of
build metadata will be written to the builds/ directory, which looks
like the following (using the ubuntu-14.100i386 template as an example):

{
  "name": "ubuntu-14.10-i386",
  "version": "2.0.20150528211301",
  "build_timestamp": "20150528211301",
  "git_revision": "6b23dd8d8ff0fb9cc4473f510bc3c54f0b415d1b",
  "box_basename": "chef__ubuntu-14.10-i386-2.0.20150528211301.git.6b23dd8d8ff0fb9cc4473f510bc3c54f0b415d1b",
  "atlas_org": "chef",
  "arch": "32",
  "template": "ubuntu-14.10-i386",
  "md5": {
    "chef__ubuntu-14.10-i386-2.0.20150528211301.git.6b23dd8d8ff0fb9cc4473f510bc3c54f0b415d1b.parallels.box": "e3a18b096cddc73384f0912c3a65ebad",
    "chef__ubuntu-14.10-i386-2.0.20150528211301.git.6b23dd8d8ff0fb9cc4473f510bc3c54f0b415d1b.virtualbox.box": "106f2ca4e6da18663e7216a72dd62e56",
    "chef__ubuntu-14.10-i386-2.0.20150528211301.git.6b23dd8d8ff0fb9cc4473f510bc3c54f0b415d1b.vmware.box": "8990550bc2a0e2e7515ed3433ec54b46"
  },
  "sha256": {
    "chef__ubuntu-14.10-i386-2.0.20150528211301.git.6b23dd8d8ff0fb9cc4473f510bc3c54f0b415d1b.parallels.box": "0a0e3c9369de005a456f0cd7d94ba4d4b562d7231c11d9c5af8e40ef77131d3d",
    "chef__ubuntu-14.10-i386-2.0.20150528211301.git.6b23dd8d8ff0fb9cc4473f510bc3c54f0b415d1b.virtualbox.box": "0c23480a99294aea8f42daea2576a41820ec3bebb99a9d0a8ab72a3de1b24137",
    "chef__ubuntu-14.10-i386-2.0.20150528211301.git.6b23dd8d8ff0fb9cc4473f510bc3c54f0b415d1b.vmware.box": "9128b66ef4bae323a123fcdd0be5a598bb538f822295ab6bf043e7630a49b608"
  }
}

In addition to the "sidecar" metadata file, a trimmed down version will
is added to each Vagrant box in /etc/bento-metadata.json. Using the
example above, here is what the file would look like:

{
  "name": "ubuntu-14.10-i386",
  "version": "2.0.20150528211301",
  "build_timestamp": "20150528211301",
  "git_revision": "6b23dd8d8ff0fb9cc4473f510bc3c54f0b415d1b_dirty",
  "box_basename": "chef__ubuntu-14.10-i386-2.0.20150528211301.git.6b23dd8d8ff0fb9cc4473f510bc3c54f0b415d1b_dirty",
  "atlas_org": "chef",
  "arch": "32",
  "template": "ubuntu-14.10-i386"
}

Also note that this changes the file naming scheme of the resulting box
artifacts in an effort to host multiple builds of the same templates in
one directory while maintaining enough information about the box within
the filename itself.

Using the same example as above, the VirtualBox provider box name is:

chef__ubuntu-14.10-i386-2.0.20150528211301.git.6b23dd8d8ff0fb9cc4473f510bc3c54f0b415d1b.virtualbox.box

Which uses the following recipe to construct the filename:

• atlas_org value (default: "chef")
• double underscore, which could be later interpreted as a slash (/)
  for an Atalas box name
• name value which may or may not equal the name of the template
  (captured as the template value)
• a dash
• version value, which removes the last digit in a version string and
  replaces it with the build_timestamp (a
  Year/Month/Day/Hour/Minute/Second format in UTC timezone)
• a period
• the string "git"
• a period
• git_revision value, which will append "_dirty" if the current
  state of the git repository is not completely clean (i.e., there are
  uncommitted changes which happens in active development)
• a period
• the value of the {{.Provider}} Packer variable, being one of
  "virtualbox", "vmware", or "parallels"
• finished with ".box"

Closes #364

[fnichol]

@yzl Ready for a review (it's a bit big, but self-contained)!

@scotthain Any thoughts as well?

[scotthain]

This looks amazing. My only question (and this may be because I don't have all the context) is what is the goal of having these files? (If that's a long conversation that you and yzl have already figured out, don't worry about it!)

Either way, having the metadata inside the box itself is awesome.

[yzl]

@scotthain It's to support a build pipeline for bento boxes, similar to the omnibus pipelines where we have a stage that builds all the packages associated to an artifact, a stage that tests all the packages, and a stage that releases them.

[yzl]

👍 This is awesome!

[scotthain]

@yzl freaking awesome. That was my suspicion!