Wrong admin username in recent virtual machines forces resource recreation

After switching to ssh key provisioning in Terraform we've been using our own ssh username to provision new virtual machines. But this practice has a nasty side-effect: when a different user invokes terraform the tool wants to recreate the virtual machine because the admin username has changed. Even worse, there is no way (at least I couldn't find it) to change the admin username in Azure. Even if you do so from the "reset password" menu in the Azure panel this will still show the old one when you reload the page after the operation completed, even after rebooting the vm.

Currently the api, git and web front-end nodes in production need to be rebuilt unless someone finds a way to avoid this.

/cc @gl-infra