Document neutron vpn service properties

This adds descriptions to the properties of:
* OS::Neutron::VPNService
* OS::Neutron::IPsecSiteConnection
* OS::Neutron::IKEPolicy
* OS::Neutron::IPsecPolicy
as well as gettext wrapping for the attributes description

Change-Id: I3312cfbeeccee925e6b06b7ac061be16997bdd34
Closes-Bug: #1224182
Merge "Document neutron vpn service properties"