Specifically, the goal here is to provide a default that can use
rootwrap.
This change implements a `priv_context.init` function that allows
oslo.privsep to hook into the startup of programs using oslo.privsep.
The intention is to call this function near the top of main() - after
oslo.config is available but before anything "interesting" is performed.
In this change, this init function just allows you to set the default
"run as root" prefix for helper_command to include something like
rootwrap.
In the future, it is expected to use this same call point to do other
"early" tasks like immediately forking privileged helpers and dropping
root if already running as root.
Change-Id: I3ea73e16b07a870629e7d69e897f2524d7068ae8
Partial-Bug: #1592043
9bf606327d