**Apache Airflow version**: 2.0.2 (but problem can still be found in master)

**What happened**:

When using the SSHHook to connect to an ssh server on a non default port, the host_key setting is not added with the correct hostname to the list of known hosts. In more detail:

```python
from airflow.providers.ssh.hooks.ssh import SSHHook
import paramiko
from base64 import decodebytes

hook = SSHHook(remote_host="1.2.3.4", port=1234, username="user")
# Usually, host_key would come from the connection_extras, for the sake of this example we set the value manually:
host_key = "abc" # Some public key
hook.host_key = paramiko.RSAKey(data=decodebytes(host_key.encode("utf-8")))
hook.no_host_key_check = False

conn = hook.get_conn()
```

This yields the exception

    paramiko.ssh_exception.SSHException: Server '[1.2.3.4]:1234' not found in known_hosts

**Reason**:
In the SSHHook the host_key is added using only the name of the remote host. 
https://github.com/apache/airflow/blob/5bd6ea784340e0daf1554e207600eae92318ab09/airflow/providers/ssh/hooks/ssh.py#L221

According the the known_hosts format, we would need 

```python
hostname = f"[{self.remote_host}]:{self.port}" if self.port != SSH_PORT else self.remote_host
```

**Anything else we need to know**:

I will prepare a PR that solves the problem.

airflow/providers/ssh/hooks/ssh.py
if self.client is not None:
            self.client.close()
            self.client = None
    def get_tunnel(
        self, remote_port: int, remote_host: str = "localhost", local_port: Optional[int] = None
    ) -> SSHTunnelForwarder:
        """
        Creates a tunnel between two hosts. Like ssh -L <LOCAL_PORT>:host:<REMOTE_PORT>.
        :param remote_port: The remote port to create a tunnel to
        :type remote_port: int
        :param remote_host: The remote host to create a tunnel to (default localhost)
        :type remote_host: str
        :param local_port:  The local port to attach the tunnel to
        :type local_port: int
        :return: sshtunnel.SSHTunnelForwarder object
        """
        if local_port:
            local_bind_address: Union[Tuple[str, int], Tuple[str]] = ('localhost', local_port)
        else:
            local_bind_address = ('localhost',)
        tunnel_kwargs = dict(
            ssh_port=self.port,
            ssh_username=self.username,
            ssh_pkey=self.key_file or self.pkey,
            ssh_proxy=self.host_proxy,
            local_bind_address=local_bind_address,
            remote_bind_address=(remote_host, remote_port),
            logger=self.log,
        )