vdsm-infra: zombie-reaper refactor

In a general intent to better organize how infra libraries and utilities
are organized.

It uses zombie-reaper as a sample use case and adds documentation for it.

Change-Id: I86a7d8d2f512395c2f50e21430b9e939f553e91b
Signed-off-by: Saggi Mizrahi <smizrahi@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31301
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Tested-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>