network: reset 'configured' flags even if we keep DHCP lease and friends on reconfigure

network: reset 'configured' flags even if we keep DHCP lease and friends on reconfigure

Follow-up for 451c2baf30f50b95d73e648058c7c2348dbf0c31.

With the commits, reloading .network files does not release previously
acquired DHCP lease and friends if possible. If previously a DHCP client
was configured as not requesting DNS servers or so, then the previously
acquired lease might not contain any DNS servers. In that case, if the
new .network file enables UseDNS=, then the interface should enter the
configured state after a new lease is acquired. To achieve that, we need
to reset the flags.

With this change, the workaround applied to the test by the commit
451c2baf30f50b95d73e648058c7c2348dbf0c31 can be dropped.