Make gitlab-ci file more DRY

Closes https://gitlab.com/charts/helm.gitlab.io/issues/102 There is a lot of duplicated code (the code that conditionally build the image if it did not find it and the code that pushes if master). I couldn't find anything in the gitlab-ci.yml reference that can allow for reusable code that runs after script (the supported after_script runs in a seperate container).

Update: The build and push phases are moved to bash functions in build-scripts and reused int the gitlab-ci scripts