Add mechanism for skipping the GitLab CE/EE seed

Split from https://gitlab.com/gitlab-org/gitlab-development-kit/merge_requests/270#note_25418381

Currently the gdk install task always runs the dev:setup task in the CE (or EE) folder. Normally this is fine, but when we're performing the install for an instance intended as a secondary geo node, for example, this is needlessly wasteful and time-consuming, because we immediately delete the database data folder after the install task is completed.

It would be useful to have a mechanism to bypass the seeding step.

@brodock suggested:

The rake dev:setup is triggered by support/bootstrap-rails. We need either to move this “bootstrap” to a separate manual step or perhaps add an optional flag to the gdk install to prevent running it:

gdk install gitlab_repo=… --skip-bootstrap-rails

To which @jacobvosmaer-gitlab said:

The way the gdk command is implemented, adding a flag like that is not a natural thing to do.

It would make more sense to have a file .gitlab-rails-bootstrap.

.gitlab-rails-bootstrap:
    support/bootstrap-rails
    touch $@

Then the bypass mechanism would be to create that file before gdk install.

mentioned in merge request !270 (merged)

added enhancement label