Add the ability to use TLS to the redis-session extension

[atgitwk]

Hello,
PHP buildpack does not seem to support the automatic setting of php ini session_save_path variable for redis-sessions services with TLS.
extensions/sessions/extension.py only handles the 'tcp:' scheme.

    def session_save_path(self):
        return "tcp://%s:%s?auth=%s" % (
            self.creds.get('hostname',
                           self.creds.get('host', 'not-found')),
            self.creds.get('port', 'not-found'),
            self.creds.get('password', ''))

To use TLS, the Redis 'uri' specification indicates that the ‘uri’ parameter could be set with scheme ‘rediss://’ instead of ‘redis://’.
This 'uri' parameter could be initialized in the json binding credentials configuration provided by the service broker.
Here's a proposal PR for managing redis-session with TLS

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