**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
self.pkey = None
        self.port = port
        self.timeout = timeout
        self.keepalive_interval = keepalive_interval
        
        self.compress = True
        self.no_host_key_check = True
        self.allow_host_key_change = False
        self.host_proxy = None
        self.host_key = None
        self.look_for_keys = True
        
        self.client = None
        
        if self.ssh_conn_id is not None:
            conn = self.get_connection(self.ssh_conn_id)
            if self.username is None:
                self.username = conn.login
            if self.password is None:
                self.password = conn.password
            if self.remote_host is None:
                self.remote_host = conn.host
            if self.port is None:
                self.port = conn.port
            if conn.extra is not None:
                extra_options = conn.extra_dejson
                if "key_file" in extra_options and self.key_file is None:
                    self.key_file = extra_options.get("key_file")
                private_key = extra_options.get('private_key')
                private_key_passphrase = extra_options.get('private_key_passphrase')