Fix sql_role handling for database_sql_server_user
Fix sql_role handling for database_sql_server_user

Refactor the sql_role handling method and fix the following error:
> undefined local variable or method `sql_role`

seancoyne/database,chr4-cookbooks/database,opscode-cookbooks/database,chr4-cookbooks/database,seancoyne/database,opscode-cookbooks/database,GannettDigital/chef-database,chr4-cookbooks/database,chef-cookbooks/database,chef-cookbooks/database,seancoyne/database,GannettDigital/chef-database,opscode-cookbooks/database