pyln: Allow non-empty directory when creating node in node_factory

So far we've always cleared the node directory when provisioning the node, but
while testing some plugins we noticed that pre-generating some files in that
directory is useful. This just adds yet another flag to `get_node` that
disables deleting any existing node directory.

contrib/pyln-testing/pyln/testing/utils.py

@@ -1011,15 +1011,16 @@ def get_nodes(self, num_nodes, opts=None):
 
     def get_node(self, node_id=None, options=None, dbfile=None,
                  feerates=(15000, 11000, 7500, 3750), start=True,
-                 wait_for_bitcoind_sync=True, expect_fail=False, **kwargs):
+                 wait_for_bitcoind_sync=True, expect_fail=False,
+                 cleandir=True, **kwargs):
 
         node_id = self.get_node_id() if not node_id else node_id
         port = self.get_next_port()
 
         lightning_dir = os.path.join(
             self.directory, "lightning-{}/".format(node_id))
 
-        if os.path.exists(lightning_dir):
+        if cleandir and os.path.exists(lightning_dir):
             shutil.rmtree(lightning_dir)
 
         # Get the DB backend DSN we should be using for this test and this