Introduce source_pif_device (replacement for pif_id) to better match TF's mental model fix import issues

[ddelnano]

This PR introduce a source_pif_device argument (replacement for pif_id) to the xenorchestra_network resource to better match Terraform's mental model and fix the import issues reported in #257.

The issue with the previous resource was that the pif_id could not be determined once a network was created (a network's PIFs change as hosts are added to the pool). Rather than directly expose what the XO api requires (a pif id), this PR adds a "source device" argument, which can always be determined even after a network was created. In addition to fixing #257, this means that if a network was terraform destroy && terraform applyed the network would be recreated properly.

Testing done

• New import test verifies that the issues experienced in Importing xenorchestra_network resources does not set vif_id properly #257 will not occur
• Verify the acceptance test suite passes