introduce helpers for generating endpoint URLs
add comment about my_admin_host
calculate keystone.conf endpoint URLs in recipe not in template

This keeps all the URL calculation code in the same
file (recipes/server.rb) which makes it easier to compare the various
values involved.
calculate my_public_host earlier

so we can calculate all service URLs in the same place
for legibility
extract temporary variables for admin/public endpoint URLs
rename temporary service_URL variables

Clarify that they are versioned.
extract temporary service URL variables

so that all URLs are calculated in the same place for clarity.
extract bind_host temporary variable for legibility
add support for VIPs