This updates our 1ES Hosted Pools (#3054) to use spot VMs, like our old Virtual Machine Scale Sets did. The tradeoff is the same: they are occasionally subject to eviction due to fluctuating capacity, but they're cheaper.

To do this, we need to pass an 'enableSpot' = $true; property in create-1es-hosted-pool.ps1, and then we need to specify demands in azure-pipelines.yml.

Also, some script improvements/refactorings (as usual, apologies for mixing this into a PR with a behavioral change, but it's cleanly factored into separate commits, and updating the pool is a big enough production that I don't want to create a separate PR for this).

• Print elapsed time.
  • This is so I can notice if the guidance in the wiki is out of date. (Currently the wiki says to expect 40 minutes and this took 41 minutes.)
• Add progress status 'Creating network interface'.
• Group commands, add progress status 'Creating credentials'.
• Drop network security rules.
  • This wasn't really achieving anything, and I suspect that it has no effect with 1ES Hosted Pools (I haven't investigated this, but it may be the case that the virtual network is essentially "lost" when the VM image is generalized and captured; note that we now delete the virtual network components at the end of the script).