Add a new SDL storage API function 'is_active()'

Implement a new SDL storage API function 'is_active()' to check healthiness
of an SDL instance. For now function only validates SDL DB backend (Redis)
connection. This function can be integrated into SDL client (such as A1
Mediator) health checks and runtime initializations.

Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>
Change-Id: I8bf457b9528e39223ea65a20d422d2fab49602bb