Add libxmlsec1 as external package dependency on OS X

This is required to use pysaml2.

There are several packages related to libxmlsec1 on Debian and Fedora,
and I'm not sure which is/are required without testing, so this patch is
limited to documenting the change required for OS X.

Implements bp keystone-to-keystone-federation

Change-Id: I61586a0b6d9411136f7e2d5a3f991bee4617f355
Merge "Add libxmlsec1 as external package dependency on OS X"