Warn admin that writing to authorized_keys is disabled when attempting to rebuild authorized_keys

We should warn admins if they attempt to run gitlab-rake gitlab:shell:setup with the write to authorized_keys option unchecked. There has been some confusion (e.g. in gitlab-org/gitlab-ee#2882) even in 9.3.5.

/cc: @mkozono, @DouweM

Edited by Stan Hu