Fix for #92 and #94

# This fixes:
#92 Undefined method if "disks" config item is omitted (on 0.10)
#94 Destroying a deleted VM results in stack trace 

# Testing
## For issue #92, I manually tested with:
* `disks` unset (not present in `config.vm.define`:
  No metion of volumes
* `disks` invalid:
  ````
ubuntu@clay-test:~/test$ vagrant up --provider=openstack
Bringing machine 'claytest' up with 'openstack' provider...
There are errors in the configuration of this machine. Please fix
the following errors and try again:

OpenStack Provider:
* A disk name is required for all disk specifications.
* A disk description is required for all disk specifications.
* A disk size is required for all disk specifications.
  ````
* `disks` set to a valid disk specification:
  * volumes already present and unused:
    ````
==> claytest: re-using volume: Clay-Test disk 1
==> claytest: re-using volume: Clay-Test disk 2
    ````
  * volumes not yet created:
    ````
==> claytest: creating volume: Clay-Test disk 1
==> claytest: creating volume: Clay-Test disk 2
    ````

## For issue #94, I manually tested:

1. VM creation
2. Destruction
3. Re-running VM destruction

Upon the second destruction, I now see:
  ````
ubuntu@clay-test:~/test$ vagrant destroy -f
==> claytest: The server hasn't been created yet. Run `vagrant up` first.
  ````