Use azure.vm.name attribute in both Azure detector and SFx hostid logic

[pmcollins]

Description: The Azure resource detector writes the "name" value from the Azure metadata API to the host.name attribute of the current resource. The problem with this is that the the host.name attribute is also written to by the system detector, and the Azure detector may not have precedence, in which case the Azure metadata name value won't be available downstream. The signalfx exporter requires the name from the Azure metadata API to operate (and other components may as require it as well at some point) so this change both saves this value to a new resource attribute key, azure.vm.name, (in addition to continuing to write it to the host.name attribute) and reads from this new attribute in the signalfx exporter.

Link to tracking Issue: #12779

Testing: Unit tests added. Manual testing was performed on Azure.

Documentation: Field added to readme

[dmitryax]

@pmcollins Can you please submit addition of azure.vm.name as a separate PR?