Fix: Redis driver does not fall back to default port

The Redis driver requires a port in the URI string read from the
Zaqar configuration. However, it is common practice to fall back
to a default port in connection URIs, so this patch implements
support for that in the spirit of the Principle of Least Suprise.

Change-Id: I3205890bbd10b49b1be59f4465617214881e6cc0
Merge "Fix: Redis driver does not fall back to default port"