First implementation of Heat Stacks

[julienvey]

This PR implements a simple workflow for heat stacks.

vagrant up => create stacks
vagrant destroy => delete stacks

Current format of a stack is

os.stacks = [{
   name: 'stack_name',
   template: '/path/to/my/template.yml'
}]

but other attributes should be added later as described in #153

Once this PR is merged, I will create sub-issues for all heat related work, so that we can split the work and have people contribute.

[ggiamarchi]

undefined method `each' for nil:NilClass (NoMethodError)

When there's no stack declared in the Vagrantfile. Should be fixed by

config.stacks.each do |stack|
  ...
end unless config.stack.nil?

[julienvey]

Done