Do not use global before_scripts in auto devops to increase customizability

Summary

Auto devops deploy script is currently using global before_script. When using the folling ci file

before_script:
  - echo hello
include:
  - template: Auto-DevOps.gitlab-ci.yml

before_script of the deploy step is overriden, breaking the entire pipeline.

The before script could be moved in the stages

Improvements

Global before_script wouldn't be cluttered. Extensibility and customizability would increase.

Risks

Involved components

https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/ci/templates/Jobs/Deploy.gitlab-ci.yml

Optional: Intended side effects

Optional: Missing test coverage