Update docs to reflect optional use of Managed Identity rather than needing to create a Service Principal

[stylesm]

Currently the QuickStart docs (https://docs.microsoft.com/en-gb/azure/cyclecloud/quickstart-install-cyclecloud) state that a Service Principal is a prerequesite to setting up a Cloud Provider Account.

This can be achieved using Managed Identity instead (a guide for which exists under the How-to section at https://docs.microsoft.com/en-gb/azure/cyclecloud/managed-identities.

The prerequesites should be changed to offer an option of assigning Managed Identity to the VM or creating a Service Principal.