Redis Transport: Small improvements of SentinelChannel

[matusvalo]

This PR contains 2 small improvements of SentinelChannel:

1. Sentinel connection string now supports default port 26379:

BEFORE:

>>> import kombu
>>> c = kombu.Connection('sentinel://localhost', transport_options={'master_name': 'mymaster'})
>>>
>>> c.connect()
Traceback (most recent call last):
  // STACKTRACE OMITTED
  File "/home/environments/kombu/lib/python3.7/site-packages/redis/connection.py", line 502, in __init__
    self.port = int(port)
TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'

AFTER

>>> import kombu
>>> c = kombu.Connection('sentinel://localhost', transport_options={'master_name': 'mymaster'})
>>>
>>> c.connect()

2. Connection with Sentinel connection string missing master_name transport option now raises ValueError with information about missing parameter:

BEFORE:

>>> import kombu
>>> c = kombu.Connection('sentinel://localhost:26379')
>>>
>>> c.connect()
Traceback (most recent call last):
  // STACKTRACE OMITTED
  File "/home/environments/kombu/lib/python3.7/site-packages/redis/sentinel.py", line 212, in discover_master
    raise MasterNotFoundError("No master found for %r" % (service_name,))
redis.sentinel.MasterNotFoundError: No master found for None

AFTER

>>> import kombu
>>> c = kombu.Connection('sentinel://localhost:26379')
>>>
>>> c.connect()
Traceback (most recent call last):
  // STACKTRACE OMITTED
  File "/home/dev/kombu/kombu/transport/redis.py", line 1154, in _sentinel_managed_pool
    "'master_name' transport option must be specified."
ValueError: 'master_name' transport option must be specified.

[thedrow]

LGTM.