Fast mode for import-live-users

[Zarquan]

The import-live-users script uses an Ansible playbook to check the CephFS mounts for each account, which can be very slow.

If all we want to do is add a new user, then a fast mode that used a simpler check, grep the fstab file, check the user's home and data directories exists, or even just check that the user exists would speed up the process.

This will become more important as we get more users.

[Zarquan]

Alternatively, if we are adding a new user, just add that user rather than iterating through all of the live users. Second thoughts, better to keep everything in the same file.