wam: add user setup script/service

Add a per-user setup script, systemd unit to run it, and add the
unit as a dependency of systemd's user-runtime-dir@.service so it
will be run before the user services on each boot.  This should
fix the various permission issues currently being seen with WAM
startup.

Bug-AGL: SPEC-3781

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Ie45b43a39530c23439332cdcd7f4550c9f6fc23c