systemd: make launchpad use the existing dbus session bus

systemd: make launchpad use the existing dbus session bus

Since the processes under launchpad_preloading_preinitializing_daemon
run as user "app", these processes should be using the dbus session bus
that runs in the systemd user session.

The simple fix is to set DBUS_SESSION_BUS_ADDRESS for
launchpad-preload.service.  Also turn this into a template service and
enable for user "app", since launchpad supports running user
applications as "developer", but no user session currently exists for
this user.

Change-Id: Ia7145b1a51c68a4ec940077193c9762496a118d2
Signed-off-by: Patrick McCarty <patrick.mccarty@linux.intel.com>