rpc: creates possibility to preserve labels on importprivkey

- changes importprivkey behavior to overwrite existent label if one
is passed and keep existing ones if no label is passed

- tests behavior of importprivkey on existing address labels and
different same key destination

test/functional/test_runner.py

@@ -157,6 +157,7 @@ BASE_SCRIPTS = [
     'feature_nulldummy.py',
     'mempool_accept.py',
     'wallet_import_rescan.py',
+    'wallet_import_with_label.py',
     'rpc_bind.py --ipv4',
     'rpc_bind.py --ipv6',
     'rpc_bind.py --nonloopback',