Add ignore-existing to create_users CLI script

With the addition of `--keep-going` to `bin/omero load` in https://github.com/openmicroscopy/openmicroscopy/pull/2699, this user/group creation script is now a potential infinite loop creator if any of the group/user exists:

```
sebastien@sbesson1:dist (12417_cli_load) $ bin/omero logout && bin/omero load  ../../omego/create_users --keep-going
Previously logged in to localhost:4064 as root
Server: [localhost]
Username: [root]
Password:
Created session 0b06fe3a-9784-4f95-ada4-4633e323bee3 (root@localhost:4064). Idle timeout: 10.0 min. Current group: system
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
Group exists: private-1 (id=3)
^CCancelled
sebastien@sbesson1:dist (12417_cli_load) $
```

With this PR, the script should ignore any existing user and finish its execution when run multiple times with `--keep-going` turned on.