strutils bool_from_string, allow specified default

In certain cases, we may want to return true by default, e.g see
the discussion in https://review.openstack.org/#/c/65725/, so provide
a "default" argument which can be used to override the existing default
False response.

Change-Id: I6c86b03125f263cf4843b4cf1addf9e9812f53a8
Merge "strutils bool_from_string, allow specified default"