Use `Gitlab.config.gitlab.host` over `'localhost'`

Use `Gitlab.config.gitlab.host` over `'localhost'`

This would fix long standing failures running tests on
my development machine, which set `Gitlab.config.gitlab.host`
to another host because it's not my local computer. Now I
finally cannot withstand it and decided to fix them once and
for all.

See merge request !7562