feat(vm_image_util): Add new build steps, update vagrant builds

This adds two new optional build steps.  The first user of these is the
vagrant images but many of the targets can be simplified now.

- fs_hook: Anything that needs to happen before unmounting the image.
  This happens after the OEM is installed but before disk images are
  made. It can be used to copy any data out of the image.
- bundle_format: Many VM types ship as some sort of archive format
  rather than plain disk images as this script originally assumed.
  Adding this final step lets us stop using the conf step awkwardly.

Vagrant now ships with a Vagrantfile and related code included in the
OEM package. This lets us version our vagrant-side code along with the
images themselves as well making the coreos-vagrant repo optional again.
The coreos-vagrant code will still be useful for handling the fancier
cluster configuration stuff but no longer has to carry the plugin code.
feat(vm_image_util): Add Vagrant metadata file

This is required for Vagrant to recognize box versions:
http://docs.vagrantup.com/v2/boxes/versioning.html
maint(set_lsb_release): Remove gentoo-release (again)

We now require Vagrant 1.5.0 both in documentation and in code which
uses os-release instead of gentoo-release.
fix(common): Switch to os-release style names in version.txt

The existing version.txt is kinda annoying. The common case of referring
to the current version requires joining three values and the names of
those values only make sense in ChromeOS. Instead just use version as a
string, using VERSION, VERSION_ID, and BUILD_ID just as they appear in
os-release. It is up to the few scripts that need the individual parts
to break the version apart.

The old values remain for the sake of compatibility.
fix(vm_image_util): Fix building vagrant vmware fusion images.
Merge pull request #236 from marineam/vagrant

Vagrant related script updates