Packetbeat RPM config file should be marked as config

By adding `%config` in the spec file infront of all config files. RPM will ensure that locally changed config files will not be overwritten when the package is upgrade.

Config files in a package can be listed with:

```
$ rpm -ql --configfiles packetbeat
$ 
```

See:
http://www.rpm.org/max-rpm/s1-rpm-inside-files-list-directives.html
