Check for ossec download

Added a check to see if the ossec archive already exists and if it does to skip downloading again. This will also allow admin with slow internet connections to download the ossec archive separately and place it in the securedrop_repo directory without the playbook failing.

Also moved the generic ossec variables to the group_vars/securedrop.yml config file.
Should we also confirm that the hash of the ossec archive matches what we expect? Otherwise, any file at that path will trigger skipping the download.