Leverage namespace package

[reyang]

Please refer to census-instrumentation/opencensus-python#445 and census-instrumentation/opencensus-python@ff80d11.

We would want to have all the integrations following the same pattern:

1. Package name: opencensus-ext-redis.
2. Namespace: opencensus.ext.redis.

[odeke-em]

Thanks for filing this issue @reyang!

Unfortunately I don't have access to publish things under opencensus namespaces. Wouldn't it make more sense to handover this repo to the folks with that access and then they can complete this? @bogdandrutu @mayurkale22 @c24t