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

Structure only

See merge request gitlab-com/migration!171

Revert "Merge branch 'database_wrangler_automation' into 'master'"

See merge request gitlab-com/migration!172

This reverts merge request !170

Database wrangler automation

See merge request gitlab-com/migration!170