Add log_daemon attribute configuring varnishlog service.

Hey guys,

We encountered a problem with having varnishlog enabled in our live systems (filling up the disk space quickly with big traffic). 

We disabled it in our wrapper cookbook, but I think it would be nice to have a possibility of disabling that on a community cookbook level using attributes.

Any feedback regarding the pull request is appreciated. 

Thanks,
Kamil & Krzysztof