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

This reverts commit e80dba7448e5b259cf95d63e7b6ba827cadd97d1.

Conflicts:
	vcloud-launcher.gemspec
Update to vcloud-core 0.15.0

We need Core::IndependentDisk#delete for destruction of our
independent disk fixture.
Add helpers to create/destroy independent disks

Specifically for creation and subsequent removal of IndependentDisk
fixtures for the launcher 'attach independent disk' feature.
Create/destroy independent disk fixture during tests

We should not expect the developer to have to create and manage a
long running IndependentDisk purely for our test - as these have
cost.

Instead, create a fixure IndependentDisk, and then remove it at
the end of the test.

NB: This hence removes the requirement to specify
    'existing_independent_disk' to vcloud_tools_testing_config, so
        it has been removed from the template.
Introduce a delay between Vapp and Disk delete

There appears to be a bug in vCD that does not fully register
Independent Disks as detached when a vApp is deleted. After a 15-20
second delay, the disk is marked detached as expected, but during
that period any attempt to remove it will fail with an 'disk is attached'
error.

NB: The #attached_vms call does not help - it returns empty immediately.
Merge branch 'master' into attach_independent_disks_redo

Conflicts:
	vcloud-launcher.gemspec
Update CHANGELOG
Merge pull request #84 from mikepea/attach_independent_disks_redo

Attach independent disks (redo of #75)