Update to vcloud-core 0.15.0
Update to vcloud-core 0.15.0

We need Core::IndependentDisk#delete for destruction of our
independent disk fixture.

UKHomeOffice/vcloud-launcher,UKHomeOffice/vcloud-launcher,gds-operations/vcloud-launcher,gds-operations/vcloud-launcher,UKHomeOffice/vcloud-launcher,gds-operations/vcloud-launcher