Expose data_center attribute on all resources

[jimeh]

As all resources will belong to a specific data center, it makes sense to have a
data_center attribute on all resources.

The value of this resource should be the data center permalink value to keep
things simple.

I'm not sure if the attribute should be read-only, or if we should make it
writable too, allowing the user to specify the data center on a per-resource
basis. Initially my plan was to keep things simple and have the data center be
set globally for the provider, with the intention of using provider aliases for
different data centers.

However I'm coming around to the idea of making things more flexible.

The data center attribute value should always be the data center permalink. If
we make it writable, that should still remain, cause otherwise there will be
issues with state diffing.

Rough task list:

• Add data_center attribute to:
  • katapult_ip resource
  • katapult_ip data source
  • katapult_virtual_machine resource
  • katapult_virtual_machine data source
  • katapult_virtual_machine_group resource
  • katapult_virtual_machine_group data source
• Add acceptance tests to use both uk-lon-01 and us-nyc-01 data centers
  to ensure multi-DC setups work:
  • katapult_ip resource
  • katapult_ip data source
  • katapult_virtual_machine resource
  • katapult_virtual_machine data source
  • katapult_virtual_machine_group resource
  • katapult_virtual_machine_group data source