afm-unit: Fix http port multi allocation

When serveral targets were set in the widget file, the
framework allocated the same HTTP port to each. This
lead to impossibility to run all targets or to a
systemd conflict.

This should fix the issue by allocating an HTTP port for
each target installed by the widget. To achieves that goal,
the afm-unit.conf tag for the port is moved from
'#metadata.http-port' to '#metatarget.http-port'.

Bug-AGL: SPEC-2068

Change-Id: I5376d6f052e0ffc5c77cc80041528637777aed1e
Signed-off-by: José Bollo <jose.bollo@iot.bzh>