[cloudstack] add create_disk_offering request
[cloudstack] add delete_disk_offering request
[cloudstack] add disk_offering model
[cloudstack] add disk_offerings collection
[cloudstack] add disk offerings to compute, tidy up
[cloudstack] add create/delete disk_offering mocks
[cloudstack] add disk_offering model tests
[cloudstack] fix typo in declaration model
Merge pull request #1771 from dm1try/cloudstack_add_disk_offering_model

[cloudstack] add disk offering model