Speedup solidfire unit tests

The SolidFire driver has a sleep that's used in a retry loop.  This
sleep was being called every iteration of the loop though
(instead of just when the call failed).

This made SolidFire the slowest test in the unit tests, and obviously
it's just stupid anyway.  So this change fixes that and only performs
the sleep if the call actually failed.

Change-Id: I560f2fdb6bf16a6a9b26184234225349ab62e816
Merge "Speedup solidfire unit tests"