Support default values for custom pipeline variables

In the context of the recently implemented parametrized pipeline builds: BCLOUD-13737

There is no way to populate the inputs with default values. Repository and Account level variables are override and values always default to ““ (as stated in Custom Vars Documentation).

One simple and elegant way to achieve this could be to allow default values to be configured at bitbucket-pipelines.yml:

pipelines:
  custom:
    custom-build:
      - variables:
          - name: BUILD_TYPE
            default: debug
          - name: SECURE_DOMAIN
            default: somedomain.com
      - step: 
          script:
            - echo "$BUILD_TYPE build for $SECURE_DOMAIN"

I hope it makes sense. It will be very useful for us in order to avoid mistakes and failed builds, but keeping the flexibility of the project.