Service management improvements

Ability to pass in to management APIs a Session object from the requests library. This allows more flexibility for authentication, including support for oauth.  Note that this doesn't introduce a dependency on requests library, as the client is responsible for instantiating.  Also, proxy support when using requests library is not done yet.

Ability to specify the x-ms-version per management class as well as par management function, instead of the previous global setting that applied to all management apis.  This will allow revisioning individual apis without the risk of breaking existing functionality.  For now, service bus management is the only management class that targets an older x-ms-version due to extensive breakage in the newer xml.

New management apis:

- list_role_sizes
- list_subscriptions (oauth only)
- rebuild_role_instance
- delete_role_instances
- create_reserved_ip_address
- delete_reserved_ip_address
- get_reserved_ip_address
- list_reserved_ip_addresses
- add_dns_server
- update_dns_server
- delete_dns_server
- list_resource_extensions
- list_resource_extension_versions
- capture_vm_image
- create_vm_image (wip)
- delete_vm_image
- list_vm_images
- update_vm_image (wip)

Enhanced management apis:

- create_virtual_machine_deployment / add_role
  - custom data
  - additional windows unattend content
  - create from a vm image
  - public ips
  - resource extensions
  - create from remote os image
  - provision guest agent
  - dns servers
  - reserved ip

- update_role
  - resource extensions
  - provision guest agent

- create_storage_service
  - account_type replaces geo_replication_enabled (preserved for backwards compat)