Fix Vm integ test via #detach_independent_disks

PR https://github.com/gds-operations/vcloud-core/pull/130 broke the integration
tests due to an oversight with Storage Profiles.

It appears that when Independent Disks are attached, it is not possible to change
the storage profile of the attached VM. Errors occur via both the API and UI.

As a result, I have created the Core::Vm#detach_independent_disks call, which works
in the same manner as #attach_independent_disks -- takes a list of IndependentDisk objects
to detach.

The vm_spec.rb integration test has been updated to document that errors will occur if
independent disks are attached, and that they should be detached first.
Merge branch 'master' into document_sp_disk_fail

Conflicts:
	lib/vcloud/core/fog/service_interface.rb
Add Independent Disks section to README
Merge pull request #131 from mikepea/document_sp_disk_fail

Fix Vm integ test via #detach_independent_disks
Add custom fields change to CHANGELOG

Also thank external contributors as per our guidelines.
Changes tense and some minor corrections per style
The gem is called vCloud Core
Also adds detach methods

Do not need to name methods, they are very clearly named and the code speaksfor itself.
Merge pull request #132 from gds-operations/tidy-up

Tidy up prior to releasing next version
Release version 0.12.0
Merge pull request #133 from gds-operations/bump-version

Release version 0.12.0
Upgrade Fog dependency to version 1.24.0

Upgrade our dependency on Fog to versions 1.24.0 or above so as to
include the changes in this PR, which were released in 1.24.0:

  - [Error if FOG_CREDENTIAL doesn't match
    session](https://github.com/fog/fog/pull/3169)
Merge pull request #135 from gds-operations/78700334-fog-credential-ignored

[#78700334] Upgrade Fog dependency to version 1.24.0
Version 0.13.0
Merge pull request #136 from gds-operations/78700334-bump_version

Version 0.13.0
[#75691732] Document vcloud-core public methods

> According to semver http://semver.org/ we should document the API
> (lower case should). We should document this by adding Yard
> documentation to public methods. See [#73962736] for an example.
Refer to where details of entity types documented

@annashipman suggested listing the types available, but I'd rather not
do that when there are already examples in the integration spec for
these methods.
Document boolean returns for vCloud operations

Technically these methods always return `true` if they return at all,
as that's the return value from `process_task` which is the last call
used by these methods.

If `process_task` never returns it throw an error, so the return is
always either `true` or an Error.
Document query options

I have documented only the option we currently use (:filter) and the option to the query engine that is disabled (:format). I have then referred to the documentation for the Fog module that defines these options (which is pretty comprehensive) for further information.
Merge pull request #137 from gds-operations/75691732-document-vcloud-core

[#75691732] Document vcloud-core public methods