added a check to see if the OSSEC archive already exists and if it does exist then skip downloading it again.
moved defining the generic ossec variables from the app/mon host_vars to group_vars.
Ansibles stat module only supports checking the md5 sum of the file.
But did and that verification to the check to see if the ossec archive already exists

http://docs.ansible.com/stat_module.html
Merge pull request #646 from dolanjs/check-for-ossec-download

Check for ossec download