Move CooldownMixin into separate file

CooldownMixin is a class that can be shared between OS and AWS
auto-scaling implementations.  This patch moves it to a shared
subdirectory.

Implements: partial-blueprint reorg-asg-code
Change-Id: I40e470460e84944d317f5b1772fc562abf62896d
Merge "Move CooldownMixin into separate file"