**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
"""
        Creates appropriate paramiko key for given private key
        :param private_key: string containing private key
        :return: ``paramiko.PKey`` appropriate for given key
        :raises AirflowException: if key cannot be read
        """
        for pkey_class in self._pkey_loaders:
            try:
                key = pkey_class.from_private_key(StringIO(private_key), password=passphrase)
                
                
                key.sign_ssh_data(b'')
                return key
            except (paramiko.ssh_exception.SSHException, ValueError):
                continue
        raise AirflowException(
            'Private key provided cannot be read by paramiko.'
            'Ensure key provided is valid for one of the following'
            'key formats: RSA, DSS, ECDSA, or Ed25519'
        )