postgresql_server: update for modern Fedora
    
    Looks like this role hasn't been used on a Fedora box for a
    while so things are kinda broken. Re-arrange all the package
    install sections to be together, use newer package names on
    Fedora (the Fedora and EL >= 8 sections are identical for now
    but I figured I'd keep them separate in case that changes), and
    use the newer config file, not the older one, on Fedora.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>