feature #1741: escape spaces instead of deleting them in x509 auth
feature #1741: test every dn in x509 authentication

This patch was provided by Boris Parak <256254@mail.muni.cz> in
http://dev.opennebula.org/issues/1741

Minor cosmetic changes
feature #1741: escape dn in X509CloudAuth.rb
feature #1741: add method each_with_xpath to object pools
feature #1741: filter possible user matches using xpath
Merge branch 'feature-1741'