wrap SSH client setup in a function

This makes it easier to monkeypatch to earlier versions. With this
patch, I can copy-paste the setup_ssh_client() function into my code
and monkeypatch older fabric versions with:

    # hack to fix Fabric key policy:
    # #2071
    def safe_open(self):
        SaferConnection.setup_ssh_client(self)
        Connection.open_orig(self)

    class SaferConnection(Connection):
        # this function is a copy-paste from #2072
        def setup_ssh_client(self):
            # [...]

    Connection.open_orig = Connection.open
    Connection.open = safe_open

This is otherwise a noop.

fabric/connection.py

@@ -456,18 +456,8 @@ def __init__(
         self.connect_kwargs = self.resolve_connect_kwargs(connect_kwargs)
 
         #: The `paramiko.client.SSHClient` instance this connection wraps.
-        client = SSHClient()
-        if self.default_host_key_policy is not None:
-            logging.debug('host key policy: %s', self.default_host_key_policy)
-            client.set_missing_host_key_policy(self.default_host_key_policy())
-        known_hosts = self.ssh_config.get('UserKnownHostsFile'.lower(),
-                                          '~/.ssh/known_hosts')
-        logging.debug('loading host keys from %s', known_hosts)
-        # multiple keys, seperated by whitespace, can be provided
-        for filename in [os.path.expanduser(f) for f in known_hosts.split()]:
-            if os.path.exists(filename):
-                client.load_host_keys(filename)
-        self.client = client
+        self.client = SSHClient()
+        self.setup_ssh_client()
 
         #: A convenience handle onto the return value of
         #: ``self.client.get_transport()`` (after connection time).
@@ -479,6 +469,21 @@ def __init__(
         #: inline.
         self.inline_ssh_env = inline_ssh_env
 
+    def setup_ssh_client(self):
+        if self.default_host_key_policy is not None:
+            logging.debug("host key policy: %s", self.default_host_key_policy)
+            self.client.set_missing_host_key_policy(
+                self.default_host_key_policy()
+            )
+        known_hosts = self.ssh_config.get(
+            "UserKnownHostsFile".lower(), "~/.ssh/known_hosts"
+        )
+        logging.debug("loading host keys from %s", known_hosts)
+        # multiple keys, seperated by whitespace, can be provided
+        for filename in [os.path.expanduser(f) for f in known_hosts.split()]:
+            if os.path.exists(filename):
+                self.client.load_host_keys(filename)
+
     def resolve_connect_kwargs(self, connect_kwargs):
         # TODO: is it better to pre-empt conflicts w/ manually-handled
         # connect() kwargs (hostname, username, etc) here or in open()? We're