Minor fixes in build logic

This reverts commit d4a51981.

cmake 3.14 can't find py3 installation when "python" points to Python 3 while cmake 3.9 can

Happened in https://travis-ci.org/skvark/opencv-python/builds/518179810 S1 jobs

Happened in https://travis-ci.org/skvark/opencv-python/builds/518836739 S1 jobs

Fixed typo

Fix OSX builds and migrate to Xcode 8.3

* Add a dedicated stage to each OSX build to download/build brew packages
* Remove bottle building logic and the corresponding trace output from the project build stage

* Reduce the number of command lines executed in most used functions
* List existing versions only when installing

* disable auto `brew cleanup`
* support formulae with disabled bottles
* install alongside to avoid "runtime dependents" update
  https://discourse.brew.sh/t/can-i-install-a-new-version-without-having-to-upgrade-runtime-dependents/4443

* fix built bottle & metadata cleanup when bottle is missing
* -e mode for before_cache step
* faster finish when already the latest version

* Add explanation to more obscure cmake args

This reverts commit 0e788e25.

This reverts commit 5288ba5f.