Allow configuration of SSL Ciphers

[dpkp]

Additional support for #1741 . While testing SSL connections w/ python3.7 locally, I found that the default ssl ciphers that my python3.7 install supported were not sufficient to establish a connection with my kafka broker. This PR adds a new configuration option ssl_ciphers that will be passed through to SSLContext.set_ciphers() and may allow use of ciphers that are available but not enabled by default on a particular python / openssl installation. In my case, this was DHE-DSS-AES128-GCM-SHA256 . Without this configuration, my connection attempts failed with ssl.SSLError UNKNOWN_PROTOCOL

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This change is 

[jeffwidman]

Test failure looks unrelated, but might be legit bug rather than transient error? I didn't investigate further:

____________________________ test_connection_delay _____________________________
conn = <kafka.conn.BrokerConnection object at 0x0000000004cc50f8>
    def test_connection_delay(conn):
        conn.last_attempt = time.time()
>       assert round(conn.connection_delay()) == round(conn.config['reconnect_backoff_ms'])
E       assert 49.0 == 50.0
E        +  where 49.0 = round(49.48000907897949)
E        +    where 49.48000907897949 = <bound method BrokerConnection.connection_delay of <kafka.conn.BrokerConnection object at 0x0000000004cc50f8>>()
E        +      where <bound method BrokerConnection.connection_delay of <kafka.conn.BrokerConnection object at 0x0000000004cc50f8>> = <kafka.conn.BrokerConnection object at 0x0000000004cc50f8>.connection_delay
E        +  and   50.0 = round(50)
test/test_conn.py:84: AssertionError

[dpkp]

flaky test -- i'll fix that in a separate PR