network: stop to assign UUID when reconfiguring link

This fixes the following race in reconfiguring link:
1. an interface requests UUID.
2. the interface is reconfigured and link_configure() is called.
3. sd-lldp client is started on the interface (it is enabled by default).
4. networkd acquires UUID, and get_product_uuid_handler() calls
   link_configure() for the link again.
5. link_lldp_rx_configure() fails to set ifindex for already running
   sd-lldp client.
6. the link enters failed state.