**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
"""
        Opens a ssh connection to the remote host.
        :rtype: paramiko.client.SSHClient
        """
        self.log.debug('Creating SSH client for conn_id: %s', self.ssh_conn_id)
        client = paramiko.SSHClient()
        if not self.allow_host_key_change:
            self.log.warning(
                'Remote Identification Change is not verified. '
                'This wont protect against Man-In-The-Middle attacks'
            )
            client.load_system_host_keys()
        if self.no_host_key_check:
            self.log.warning('No Host Key Verification. This wont protect against Man-In-The-Middle attacks')
            
            client.set_missing_host_key_policy(paramiko.AutoAddPolicy())
        else:
            if self.host_key is not None:
                client_host_keys = client.get_host_keys()
                client_host_keys.add(self.remote_host, self.host_key.get_name(), self.host_key)
            else:
                pass