Some updates per User Feedback

[chelma]

Description

A conversation with some users indicated a couple pain points we can easily address:

• The README uses an --expected-traffic of 1 Gbps, which results in a large installation. We should use a smaller number for that example.
• Users expect cluster-destroy to destroy everything and are surprised when it doesn't. It is un-obvious to them that they need to supply the --destroy-everything flag to get that behavior. Per @awick suggestion, we can address by adding a --retain-traffic-data flag and requiring users to supply one of the two flags.

[chelma]

Code to address merged; resolving.

[chelma]

Oops - code hasn't been reviewed/merged yet. Reopening 😄

[chelma]

Ok - code has been approved/merged now. Resolving again. 😉