The Azure Cloud Shell is a handy new addition to the Azure Portal.
Currently, you have access to a bash shell with the Azure CLI tooling (and some other stuff), and a PowerShell option is in limited preview.
I've needed this a couple of times (with some infrastructure testing) and using the Cloud Shell, I could restart all the VMs in my subscription with
az vm restart --ids $(az vm list --query ".id" -o tsv)
If you want to limit it to just one resource group, you could use
az vm restart --ids $(az vm list --resource-group YourResourceGroupNameHere --query ".id" -o tsv)