External url variable should match

This is to illustrate a fix for https://gitlab.com/gitlab-org/omnibus-gitlab/commit/0402cdc9015bfd9bd516b021a2fff63ac99ebea1

In the old situation:

1. The variable contained the wrong thing (named external url, contained an FQDN)
2. The default value would lead to http begin added again

THIS MR IS NOT TESTED, just an illustration of the proposed fix

Thanks for the MR!

postinst has been reworked to accommodate the config template in !261 (merged) so I will close this in favour of 261.

Status changed to closed

OK, thanks