Use global variable for specifying builder image revision

Currently, if we bump a revision in builder image tags, we have to change it in 18 places in our .gitlab-ci.yml file. If we use a global variable for this, we can simply change in one place and use that everywhere.

Yes, there may be situations where a selected subset of builds should require an older/different revision. Still, it is safe to assume the size of that subset will be relatively less. So, the number of replaces will also be effectively less.

This MR does the following

• Use a global variable for setting revision
• Use revision for jessie_packer and ruby_docker images also
• Bump revision to 0.0.4 which was pushed recently - https://dev.gitlab.org/cookbooks/gitlab-omnibus-builder/pipelines/47760