default config_dir broken on FreeBSD

[lesinigo]

On FreeBSD the file templates/consul.freebsd.erb hardcodes the config directory as /usr/local/etc/consul.d, but then manifests/params.pp still has /etc/consul as default and the module ends up writing the configuration there.

params.pp should be modified to use /usr/local/etc/consul.d as default on FreeBSD systems, which is the preferred and correct path on that operating system.

Also, all configuration templates like templates/consul.freebsd.erb should use ::consul::config_dir instead of hardcoding values, to avoid having the module writing config in one place and consul reading it somewhere else.