Move registration of glance service user to common recipe

There's no need to have that code done twice for both api and registry,
which was also causing a CHEF-3694 bug (since the resources had the same
name).

WARN: Cloning resource attributes for keystone_register[register glance user] from prior resource (CHEF-3694)
WARN: Previous keystone_register[register glance user]: /var/chef/cache/cookbooks/glance/recipes/api.rb:108:in `from_file'
WARN: Current  keystone_register[register glance user]: /var/chef/cache/cookbooks/glance/recipes/registry.rb:85:in `from_file'
WARN: Cloning resource attributes for keystone_register[give glance user access] from prior resource (CHEF-3694)
WARN: Previous keystone_register[give glance user access]: /var/chef/cache/cookbooks/glance/recipes/api.rb:118:in `from_file'
WARN: Current  keystone_register[give glance user access]: /var/chef/cache/cookbooks/glance/recipes/registry.rb:95:in `from_file
Merge pull request #94 from vuntz/fix-CHEF-3694

Move registration of glance service user to common recipe