Bug 1558957: adjust shipit groups
Bug 1558957: adjust shipit groups

* Removed unused signoff.shipit.staging.mozilla-releng.net
* Added the `vpn_cloudops_shipit` LDAP group to
  `application.authorized_groups` for shipit (prod and staging). Ship-it
  performs additional checks in order to use refined permissions.
  Additionally the API backend allows only IP addresses from the
  `vpn_cloudops_shipit` VPN IP addresses.

mozilla-iam/sso-dashboard-configuration,mozilla-iam/sso-dashboard-configuration,mozilla-iam/sso-dashboard-configuration