Add Sentinel configuration reading

Added reading Sentinel configuration from environment variables.

Sentinel address is configured via 'DBAAS_SERVICE_HOST' and
'DBAAS_SERVICE_SENTINEL_PORT' environment variables.

Sentinel master name is configured via 'DBAAS_MASTER_NAME' environment
variable.

Sentinel support is now activated.

Using Sentinel is optional. If Sentinel related environment variables
are not set, then static address for Redis server is used as before
(i.e. this change is backward compatible).

Signed-off-by: Rolf Badorek <rolf.badorek@nokia.com>
Change-Id: I34031ac56268673996c6fecb00eeed6e5dda8b9a