Fix stackforge_libs installation step

When stackforge_libs is enabled, the WSME and Pecan libraries
are checked out from stackforge and installed from source instead of
pip. This change introduces a new function to perform the installation
without attempting to sync the global requirements list, since the
version of setup.py in the global requirements repository breaks the
dependencies for WSME (there is no ipaddr library in python 2, so we
need to install it, but under python 3 where it is part of the stdlib we
cannot include it in the requirements).

Fixes bug 1252488

Change-Id: I58357757ac67a919bf70178b76f65fa0a9e16242
Merge "Fix stackforge_libs installation step"