Catch loading failures if transport_url is not set

The current implementation always tries to get a transport from
oslo.messaging assuming the transport_url option has been set. This is
done to keep backwards compatibility. However, since the default
`rpc_backend` is rabbit, it'll always try to load such driver. The
problem raises when `kombu` is not installed and the `notifier_strategy`
is set to qpid. This will make glance-api fail because it'll try to load
the rabbit driver *before* loading the qpid one.

This patch puts the first load attempt in a try / except block. The
error caught (if any) will be re-raised *just* if transport_url has been
configured, otherwise it'll move forward and try to load the notifier
driver using the old configuration options.

The patch also removes an obsolete test.

Change-Id: I00e653704463ea8245e2960050e0d5e6839f278a
Closes-bug: #1302661
(cherry picked from commit 164d1ab46f06141e2c12b944e15ae10f20dace38)
Merge "Catch loading failures if transport_url is not set" into milestone-proposed