Clarify non-manual nature of performance tests

[jseldess]

Jesse Seldess commented:

Mention that we use internal tools like roachprod to automate the provisioning, etc.

Jira Issue: DOC-430

[jseldess]

@rmloveland, when you update the benchmarking instructions, you might want to find a place to mention that we use internal tools to automate a lot of the work.