Add pexpect to global_requirements

The Trove guestagent requires the pexpect>3.1 python package
which doesn't exist in in the global requirements today.
Adding this package requirement to the global requirements.
The library was tested on: Python 2.6.9, Python 2.7.3, and
Python 3.2.3

Change-Id: I1e8e9e3ccb4e7ea39890620daa182e15efbd7c12
Merge "Add pexpect to global_requirements"