Add retrying lib from global requirements

Add retrying lib from global requirements

Submitted a retry decorator and it was pointed out that
the retrying library was already in global-requirements,
it would probably be good to leverage this and use it rather
than roll our own, so let's add it to our version of requirements
and then we can look at whether using a decorator or just
leveraging the lib is the right way to go.

Change-Id: Ieea042e5cd65b7c7bfa803996f60856fcae054bf