Add parameter to allow to skip auto migrations

[slinger987]

In the case of an HA setup, there is no need to run the auto migrations on all servers and the documentation specifies that the /etc/gitlab/skip-auto-migrations file sshould be created to prevent database backups on an RPM upgrade. There is an option to prevent the rails to perform the migrations with 'gitlab_rails['auto_migrate'] = false' in gitlab.rb, but this doesn't help during the package upgrade. More information on the HA setup documenation:

https://docs.gitlab.com/ce/administration/high_availability/gitlab.html

[tobru]

Would you mind to create a PR?

[davidgubler]

49f900b adds this functionality.