net: lib: lwm2m: Don't create server object in bootstrap

When bootstrap is used, the server object shouldn't be autocreated.
Automatically creating object may cause problems after bootstrap
has been done and bootstrap server deletes and creates instances
for server object. In the next boot the auto-created server object
may have clashing server_id with the server object that the
bootstrap-server has created.
Also lifetime wasn't properly added to the registration message from
the server object.

Signed-off-by: Jarno Lamsa <jarno.lamsa@nordicsemi.no>