Adds the random option to cinder retry function

Adds the random option to cinder retry function

The current retry functionality in Cinder, always uses fixed
numbers to define the time to wait before performing the next
attempts. This behaviour can make the retries of multiple
requisitions to collide in each attempt as they will wait the
same amount of time and try to use the same resource together
again.

This patch changes the behaviour of cinder.utils.retry() to
receive a parameter that allows the function to implement
randomly generated wait intervals.

Change-Id: If746434e4a19895d649529e9e9f267410a1a1c48
Closes-bug: 1486142