Fix default ports.

[pszulczewski]

When using nautobot-nornir the default value of device port is None.

In [16]: nornir_obj.inventory.hosts['3750-x'].dict()
Out[16]: 
{'name': '3750-x',
 'connection_options': {'netmiko': {'extras': {'secret': None},
   'hostname': None,
   'port': None,
   'username': None,
   'password': None,
   'platform': 'cisco_ios'},
    ...

This causes an error on int(port) in the constructor:

nautobot_os_upgrades-worker-1    |   File "/usr/local/lib/python3.8/site-packages/pyntc/devices/ios_device.py", line 65, in __init__
nautobot_os_upgrades-worker-1    |     self.port = int(port)
nautobot_os_upgrades-worker-1    | TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'

This PR fixes this behavior, by allowing port=None to be passed, but still falls back to the default value of 22.
Port default value in the constructor has been changed fro 22 to None for uniformity with other drivers.
This change also eliminates the need to set ports in nautobot plugin configuration options.

[jeffkala]

can you drop py3.7?

[pszulczewski]

can you drop py3.7?

Yes, just realized that a while ago. It's done.