GitLab

Use bash -n to check script sanity instead of actually executing it

Enable migration to use script files and script functions.
Created scripts to execute steps inside scripts folder tree.
Merged production and staging steps and added support to specify a role
(associated to steps_$role).
Sourced source_vars into migration script.

database_wrangler_master

Conflicts:
	.gitlab/issue_templates/failover.md

More symlink fixes

See merge request gitlab-com/migration!175

Harden the scripts against symlinks

See merge request gitlab-com/migration!174