Support SoftLayer disk_capacity configuration

[hugespoon]

This adds support for specifying the disk_capacity hash in your .kitchen.yml in the form of the following:

---
driver:
  name: vagrant
  provider: softlayer
  customize:
     disk_capacity:
       "0": 25
       "2": 100

We need to prevent enclosing the hash in quotes due to the conversion to a symbolized_hash. The above would result in the following invalid Vagrantfile otherwise:

p.disk_capacity = "{:"0"=>25, :"2"=>100}"

[fnichol]

@hugespoon Thanks for the disk_capacity tweaks! I took the liberty of rebasing this against master (which now has specs) and added a test to ensure that we don't regress in 706256a. Thank you!