add cheaper_busyness plugin to default build profile

[prymitive]

I've been using it since first release and it works really well, it would be nice to have it in default build profile.

[anthonyrisinger]

i'd throw an ACK at this, whatever it's worth. we deployed it throughout this last (US) tax season and it handled nonstop variation between full-stop <=> continuous-onslaught rather beautifully... traffic was gobs of SOAP/ReST API (server+client), static content, Django everywhere, and some smaller frameworks/XHR-ness/etc i forget the names of.

...at any rate, this plugin's more "anticipatory" style ("keep more than enough hot to handle the expected-immediate-load" vs. "oh !@#$! please wait [block] while i buckle my pants") better matches the similar expectations/latencies/perf we hold of our production stacks (eg. "hot 'n ready")

[unbit]

It's ok for me, but please review docs as i do not know if they are updated with latest additions

[prymitive]

I need to document 2 backlog related options.