Use Cheffish resources for user and org creation

[jtimberman]

As part of a larger effort to make use of our own primitives, the
test::post-install recipe should use Cheffish chef_user and
chef_organization resources to create the example user and organization.

It may be many more lines of code, but at the end of the day it is
declarative, and these resources are inherently both convergent and
idempotent. Running the chef-server-ctl commands requires a guard, which
parses output that can change more easily in a user-facing tool. The
Cheffish resources use the REST API of the Chef Server, and changes to
the API itself is made with far more consideration.

That said, there's a bug in the Cheffish chef_organization resource,
addressed by the following pull request:

chef/cheffish#50

As such, we install the Cheffish gem built from the branch in that PR.
When that is merged and a new Cheffish gem is released, then we can
remove the git and rake use in the recipe, and just use chef_gem.

[jtimberman]

As it turns out there's a race to compile time in the post-install recipe to get git installed on RHEL platforms. Going to revisit this perhaps elsewhere :/.