Allow QA_IMAGE to be passed to manual job/pipeline

From gitlab-org/gitlab!153085 (comment 1908563126):

we'll also need to update the gitlab-ci.yml file here to be something like this in your Charts MR

    - if [ -z "$QA_IMAGE" ]; then
    -   if [ -n "$QA_GITLAB_REVISION" ]; then
    -     QA_IMAGE="registry.gitlab.com/gitlab-org/gitlab/gitlab-ee-qa:$QA_GITLAB_REVISION"
    -   else
    -     QA_IMAGE="gitlab/gitlab-ee-qa:nightly" 
    -   fi;
    - else
    -   QA_IMAGE=$QA_IMAGE
    - fi;

This should set the QA_IMAGE to what is being passed to the pipeline, manually.