Added warning for trusted_domains after CLI upgrade

If trusted_domains is not set after a CLI upgrade, show a warning in the
output.

Note: the CLI upgrade will not set "trusted_domains" to "localhost" any more as it doesn't make sense. "localhost" is always considered as trusted internally.
This makes it possible, for each upgrade, to show the warning again if the admin always forgets to set the trusted_domains...

Fixes #7663

Note: there will be a code conflict with https://github.com/owncloud/core/pull/7611 in which I'm still 

Please review @LukasReschke @karlitscheck