Allow copying instead of moving existing data during restore

Allow a path to the restore task to which to copy any existing content in data directories.

When passing this path to the restore action, all contents of the gitlab-data directories should be copied to the specified location. After that we can empty the original directories before the restore.

This would avoid issues when the data-directories are mount points.

This issue was brought up in https://gitlab.com/gitlab-org/gitlab-ce/issues/19309#note_69550356