Document performance testing of cloud site

We should provide guidance/example on how to assess the performance of a cloud site, it would be useful to understand what can be expected from a site and also to help diagnose/troubleshoot deployment problems.

Some pointers:
- https://github.com/GoogleCloudPlatform/PerfKitBenchmarker
- https://cloud.google.com/compute/docs/benchmarks-linux
- https://phoronix-test-suite.com/

Example:
```sh
# Using sysbench
sudo apt-get install sysbench
sysbench --test=cpu --cpu-max-prime=20000 run
# Using phoronix test suite
sudo apt-get install phoronix-test-suite
phoronix-test-suite list-available-suites
# Chose one, and run it.
phoronix-test-suite run pts/cpu
```