Add retrying to global-requirements

Retrying is an Apache 2.0 licensed general-purpose retrying library,
written in Python, to simplify the task of adding retry behavior.

Retrying would be useful for the following projects:
- glance
- oslo
- nova
- certainly more...

Useful in the context of:
- session timeout
- retry on specific exception
- etc.

Change-Id: I11c20997d6b7ce37943b55c759845a3e655c84d3
Merge "Add retrying to global-requirements"