Mattermost and GitLab external URL's are not configured properly
This MR resolves the following issues:
- Mattermost and GitLab URL's incorrectly configured
- GitLab now uses an external EBS volume for Git data, and re-attaches it if the GitLab node fails. (Note that this only works within a single AZ)
- Delay startup of GitLab Runner so that it is able to successfully register
cc @marin
Closes #1