Fixes #35650 - write correct form of deb repo

In Host registration -> advanced its possible to enter a debian
repository which will then be written to
/etc/apt/sources.list.d/foreman_registration.list
If you enter a debian repo like "deb https://example.com buster stable",
shell_escape method would rewrite the string so that it looks like:

deb\ https://example.com\ buster\ stable

This repo config can not work. Use save_to_file to write the file.