improve 7.3 upgrade process & redis setup

Address improvements discussed in https://github.com/gitlabhq/gitlabhq/commit/2b35c887904b6a7a05a7c88fdf1ccd66ba943e20#commitcomment-7829537, https://github.com/gitlabhq/gitlabhq/commit/5704e6fee9d2dea67c4a91b43d06a9958484d29b#commitcomment-7829367, & https://github.com/gitlabhq/gitlabhq/commit/5704e6fee9d2dea67c4a91b43d06a9958484d29b#commitcomment-7829381