[vsphere] allow to create a vm with multiple disks.

unitNumber has to be unique.
[xenserver] added create_sr request (create Storage Repository)
[xenserver] added destroy_sr request (destroy Storage Repository)
[xenserver] added unplug_pbd request
[xenserver] added new requests to compute service

- unplug_pbd
- destroy_sr
- create_sr
[xenserver] added unplug method to PBD model
[xenserver] missing Pool model attribute, new methods

- Added missing suspend_image_sr attribute
- added default_sr getter/setter
- added suspend_image_sr getter/setter
- added generic set_attribute method
[xenserver] added new methods to StorageRepository model

- destroy/save operations
- generic set_attribute method
[xenserver|docs] Added new example that covers a Citrix KB ctx116324 article

Translated http://support.citrix.com/article/ctx116324 to fog/xenserver
[docs::aws::storage] added [] to make it pretty
[docs::aws::storage] yard doc syntax fix
[docs::aws::storage] reformatted copy_object
Merge pull request #1433 from westonplatter/aws_docs

[docs::aws::storage] fixed some syntax and formatted a request
[docs::aws::storage] reformatted delete requests
Merge pull request #1437 from ohadlevy/vsphere-multi-disk

[vsphere] allow to create a vm with multiple disks.
Merge pull request #1436 from westonplatter/master

[docs::aws::storage] reformatted fog/aws/storage/requests delete
adding support for rackspace loadbalancer content caching
Merge pull request #1438 from luizfar/add_content_caching_to_load_balancer

adding support for rackspace loadbalancer content caching
[docs::aws::storage] standardized return format
  for key with values,
    # @return variable [data_type]:
  for values,
    # * variable [data_type] - description of value
[docs::aws::storage] WIP on get requests