Rewrite nginx upstream template in 'nginx' role

- nginx upstream template has been cleaned up and now has more ways to
  specify server addresses, including using host groups directly from
  inventory;

- you can now specify which upstream template to use using 'item.type'
  variable in nginx upstream definition;

- PHP5 upstream support has been moved to a separate template;
Rewrite of 'sks' role

'sks' role has been cleaned up and streamlined. It currently uses
'ginas_sks' Ansible group to automatically manage a cluster of hosts as
SKS keyservers with the first one being a web server frontend to the
service.
Update CHANGELOG.md
Add missing parameter to default nginx upstream
Merge pull request #130 from drybjed/role/sks

Updates in 'sks' and 'nginx' roles