config time zone

Instruct user to set time zone option that was added in https://github.com/gitlabhq/gitlabhq/pull/8015
I understand that setting the timezone will be nice for some GitLab installations. However, considering that the default (UTC if I understand correctly) is sane, I think this settings change should be presented as an optional step, not a necessity. Manual GitLab updates are daunting enough as it is so I believe we should think twice before telling users that they _have_ to take an extra step.