Use correct credentials when pushing from weblate-gatekeeper (gitlab-migration#74)

Pushes from weblate-gatekeeper should be done as the
"role-weblate-gatekeeper" GitLab user, and for that we need to use a
specific SSH key when pushing.

This commit adds a new "Host" in gitolite's SSH config and changes the
remote URL of the gatekeeper repo to use the that new host, thus making
use of the correct key to connect to GitLab.

See: https://salsa.debian.org/tails-team/gitlab-migration/-/issues/74