Revert "Merge pull request #75 from mikepea/attach_independent_disks"

This reverts commit c1ec4d7ae3104e50beaaec4f4bb747c7fa970825, reversing
changes made to 9858393ee27671f1ac4dc6560b8feb36e69254d4.

Reverting this as it's currently preventing our integration tests from
running without us manually creating an independent disk for the
purposes of these tests.

These changes can be merged back into `master` once the set-up and
tear-down of independent disks is handled automatically.

/cc @mikepea @annashipman
Merge pull request #80 from gds-operations/revert-independent-disks-for-now

Revert "Merge pull request #75 from mikepea/attach_independent_disks"
Upgrade dependency on vCloud Core to 0.13.0

Upgrade dependency on vCloud Core to 0.13.0. An error will now be raised
if your `FOG_CREDENTIAL` environment variable does not match the
information stored against a vCloud Director session referred to by
`FOG_VCLOUD_TOKEN`, so as to guard against accidental changes to the
wrong vCloud Director organization.
Release version 0.5.0
Merge pull request #81 from gds-operations/78700334-upgrade_core

[#78700334] Release version 0.5.0