Cleanup the tempest readme

This commit makes several needed cleanups and improvements to the
tempest readme to make things clearer and elaborate on some sections.
Also as the readme is intended to be a tempest users guide, this moves
sections which are more developer focused to the hacking guide.

Change-Id: I4180ce18268443873fe7d8d5e7d0aaebc2cd1e08
3 files changed