abort if o/s is not rhel family

we should probably actually handle it properly, but in the meantime this
should prevent stranger errors if someone runs the playbook on debian or
anything else other than rhel
add postgresql support to ansible playbook

setting the new "db_platform" variable controls whether to install using
mysql or postgresql as the database storage platform.
fix reference to renamed file
ensure correct permissions on /etc/ratticweb.cfg
typo in postgresql service name
fixes for postgresql ansible playbook
fix ordering of tasks for dependency resolution