converge binaries used in integration tests and examples

[ryane]

Currently, any integration test (forthcoming in #355) or examples (elk, swarm)
that use the Vagrant or Terraform provisioners are tied to the latest official
release of converge. This means that they will break if they are updated to take
advantage of new unreleased converge features. The only workaround at this point
(afaik) is to upload a locally built converge binary to a public url somewhere
and then build a custom version of the provisioner that points to that location.

It would be nice if we had a way for users and developers to run the tests and
examples using a locally built converge.

Potential solutions:

• update Terraform and Vagrant provisioners to allow sourcing the converge
  binary from a location on the local filesystem
• publish nightly releases to a public location and support configuring the
  provisioners to point there
• Other ideas?

[ryane]

note this affects, at minimum, #346, #355, and #334 (just the elk example part of it)