Add decorator to the global requirements

This module is usefull for declaring decorators. It is important in some
cases where method signature has to be preserved between the
original method and the decorated method ; when inspect is used.

We have a feature in rally requiring to preserve signature of the
original method. Here is a link which explains why we need this
dependency: http://fperez.org/py4science/decorators.html

Change-Id: Icef9a043fbf892bb401139902dc38cb6b4bf8030
Merge "Add decorator to the global requirements"