Add xmlsec1 dependency comments

Right now, Keystone IdP uses xmlsec1 command line
tool to sign the generated SAML assertions:

https://github.com/openstack/keystone/blob/master/keystone/contrib/federation/idp.py#L369

If not installed, the command will fail.

Change-Id: I9d846c4d5913b7b711f540989c243371e1da2181
Merge "Add xmlsec1 dependency comments"